Gemstones Part 3 - Non-Silicate Gems

Non-silicate minerals are much less common than the various silicates, making up just under 10% of Earth's crust. Yet, here too we find a variety of colourful gems as well as a precious mineral made up of just one element - carbon.

Spinels

Spinels are a class of minerals with the chemical formula A²⁺B₂³⁺O₄^2-. These are cubic crystals where oxygen anions form a packed lattice around cations A and B, which can one of many different transition metal ions. You can learn how transition metals create colours in gems in the first article in this series, "Gemstones - The Science of Their Colour." Here is what the arrangement looks like, with magnesium and aluminum ions in the A and B positions, below:

(NIMSoffice; Wikipedia)

The above structure is that of true spinel (MgAl₂O₄), a gem after which the mineral class is named. In general, these gems can be opaque to transparent and dull to lustrous. Hard and durable gem quality crystals come in red, blue, green, yellow or black. Both the French and British crown jewels include some spectacular spinels. Red transparent spinels were once considered rubies as they look just like them, and they are often found along with them. These gems are found several places in the world in both metamorphic rock and in certain igneous rocks rich in aluminum and magnesium. Interestingly, some of them are more rare and valuable than many of the gems they imitate.

Artificial spinel can now be made in the laboratory. These are usually the stones you see in imitation birthstone rings, which is not necessarily a bad thing as these spinels can achieve a clarity and hardness that rivals any other gemstone. Below are two uncut samples:

(S Kitahashi; Wikipedia)

Chrysoberyls

Chrysoberyls, gemstones with the formula BeAl₂O₄, are completely different from beryls, which are silicates. These gems are composed of twinned crystals giving them a hexagonal appearance, an arrangement that makes them extremely hard. Chrysoberyls are translucent to transparent and usually range from pale green to yellow.

Cat's eye gems owe their special appearance to a visual effect, called chatoyancy, caused by the reflection of light by parallel channels in the stone. Although cat's eye tourmalines and tiger's eye quartz gems are found, most cat's eye gems are chrysoberyls, shown above left. A tiger's eye is shown for comparison, below left.


(Gemshare; Wikipedia)

If a few chromium ions replace the aluminum ions in chrysoberyl, alexandrite is formed. This creates an intense absorption of yellow light and also results in an unusual effect - the stone changes colour based on ambient light. The same Russian alexandrite gem is green in (white) daylight and red under (yellow) incandescent light, shown below:

(User; Wikipedia)

Alexandrite is very rare. It forms much like other gems do but the elements beryllium (in all chrysoberyls) and chromium (only in alexandrite) usually don't occur together. They have contrasting chemical characteristics and as a result usually show up in contrasting rock types. To make an alexandrite you also need a lack of silica because if it is present, an emerald will form instead. Most alexandrite was found in Russia but those sources have all but been exhausted. However, some gem-quality deposits have recently been discovered in Brazil.

Carbonate Collector Minerals and Pearls

Pearl

Pearls are the only gems made by living animals - bivalve mollusks. They are part mineral (calcium carbonate) and part biological material (complex proteins called conchiolin).

When a microscopic intruder or parasite settles inside mollusk's mantle folds, the mollusk reacts defensively by secreting calcium carbonate and conchiolin over the irritant in successive fine layers. This secretion process is repeated many times over a few years, eventually creating a pearl. It is a wive's tale that sand grains that slip into a mollusk create pearls. The animal's immune system does not recognize inorganic materials.

Almost any mollusk can make a pearl but only one kind of pearl, called a nacreous pearl, is generally valued as a gem. This kind of pearl is made only by bivalves and clams. A nacreous pearl contains outer layers of nacre. To make nacre, thin hexagonal platelets of a crystalline form of calcium carbonate called aragonite are sandwiched between layers of a complex protein matrix containing chitin, lustrin and silk-like proteins. This construction makes pearls iridescent, strong and resilient. The mollusk continues to build the pearl layer by layer for the rest of its life. Nacre is also called mother of pearl. It is the same material that lines the inside shells of some mollusks, the nautilus for example, shown below:

(Chris 73; Wikipedia)

Gem-quality nacreous pearls are formed in some freshwater mussels and in some saltwater oysters. Pearls can be cultured by inserting either a small piece or bead made of mantle tissue into the mantle folds (or sometimes the gonads, ouch) of the animal. The mollusk will immediately begin to cover up the irritant with layers of nacre. Natural pearls have widely varied shapes, sizes and quality whereas cultured pearls can be designed to start round using beads and they are usually flawless. Each animal can form several pearls at once. More than 99% of all pearls sold are cultured pearls. Below, pearls are extracted from a pearl oyster:

(Pomakis; Wikipedia)

Dyes can be inserted into the mollusk shell (more often done with freshwater mussels) to create pearls of different colours - pink, yellow, green, blue, brown, purple or black. Pearls also naturally vary in hue depending on the type of mollusk. For example, natural black pearls come only from the black lip oyster.

The lustre of pearls is created by the reflection, refraction and diffraction of light within the fine translucent layers of nacre. Thinner layers create a finer lustre. The best pearls have an almost metallic mirror-like lustre. The overlapping of successive layers disperses incoming light as well, creating subtle iridescence.

The ornamental use of pearls is probably as old as mankind itself. You don't need to mine, cut or polish them. Just open up the right mollusk and there it is.

Pearls, being composed of calcium carbonate, must be cared for with a gentle touch. They are susceptible to attack by acids such as vinegar, perfumes, lemon juice and those in our skin. They are also very soft compared to other gems (2.5 on the Mohs scale) so any abrasives must be avoided.

Carbonate Collector Minerals

Carbonates are usually sedimentary minerals that are usually made up of calcium carbonate (CaCO₃), such as calcite (CaCO₃) and dolomite (CaMg(CO₃)₂)). These minerals tend to be much too soft to make gems, though I include them here just because they too can be so beautiful, and collectable. Calcite is a significant component of all three igneous, metamorphic and sedimentary rock types. It makes up about 4% of the Earth's crust. Calcite crystals can take on over 300 different forms as well as many twinned varieties. It is the primary material in cave formations. Mexican "onyx" (not to be confused with true onyx, a silicate mineral) and travertine are beautiful and useful examples. Both materials are susceptible to acid (that is how calcite dissolves to make various cave formations), just like other carbon-rich rocks such as (sedimentary) limestone, made of compressed coral and/or protist skeletons  and marble,  the only metamorphic carbonate rock.






To the left is a sample of large (up to 5.75 cm) calcite crystals embedded in a matrix mostly made up of another calcium carbonate - dolomite.

To the left is a stunning rhodochrosite (MnCO₃) crystal. Both this sample and the one above it were found in mines in the United States.


(both photos: Rob Lavinsky/iRocks.com)







Sulfide Collector Minerals

Sulfide minerals form when a negative sulfide ion (S^2-) combines with a positive ion such as copper Cu²⁺, lead (Pb²⁺), zinc (Zn²⁺) or silver (Ag⁺).

One example of a sulfide gemstone is Sphalerite, a zinc sulfide in crystal form. This mineral crystallizes into a cubic lattice very similar to the structure of diamond.

Transparent crystals are rare because impurities such as iron are usually present, making the mineral opaque, but they can be found in red (shown below), honey brown, orange and green, and they can display very high dispersion (fire), over three times that of a diamond, but they are soft and better treated as collector's pieces than in jewelry.

(Rob Lavinsky; Wikipedia)

Sphalerite is usually formed along with a mineral called galena, a lead sulfide, within veins and fissures where igneous and sedimentary rock meets.

Pyrite is another sulfide (FeS₂) mineral, in fact the most common one. You may have heard of it as fool's gold. It is found along with other sulfides and oxides in quartz veins, in sedimentary and metamorphic rock and in coal beds. And it can actually be sometimes found with gold.

Pyrrite can form large clean lustrous cubic crystals that are very ornamental:

(CharlesMilan; Wikipedia)

Phosphate Gems

Phosphates, in which a negative phosphate ion (PO₄^3-) combines with a variety of usually complex positive ions, tend to form minerals too soft to be gem quality. The most common phosphate mineral is fluorapatite (Ca₅(PO₄)₃F (calcium fluorophospate). Apatite, a very commonly found rock mineral, releases phosphates into soil and water as part of the phosphate geological cycle, which are taken up by animals and plants as part of their lifecycle. Phosphates are deposited in great layers in the ocean as ocean organisms die and sink, eventually turning into sedimentary phosphate-rich rock. When some of this rock then undergoes metamorphosis, intense pressure and heat can set up conditions where large well-formed crystals of fluorapatite can form.

The pure mineral is colourless but samples can be various colours thanks to small impurities, many of which are fairly hard and of gem quality (the Hyperphysics site has some stunning examples of these ores and gems). Here is a rare blue sample from Brazil:

(Rob Lavinsky/iRocks.com)

Our tooth enamel is made of hydroxyapatite, a crystalline form of calcium phosphate. Fluorinated drinking water turns some of that calcium phosphate into fluorapatite, a mineral much more resistant to acid attack.

Turquoise

Turquoise is an opaque bluish green phosphate mineral hydrate that has the chemical formula CuAl₆(PO₄)₄(OH)₈·4H₂O. Below is a tumbled turquoise pebble:

Turquoise is about as hard as glass and it is susceptible to acid, so it is often coated with clear wax or a resin before it is used in jewelry.. It has a highly variable crystal system but it never forms single well-defined crystals. It often contains impurities. The pebble above is flecked with pyrite, for example. Turquoise colours are as variable as its structure. Copper makes a more blue turquoise, while either iron or dehydration will make turquoise appear greener. Below are rough nuggets and cabochons of turquoise from the United States:

Deposits of turquoise are most often found in arid regions encrusting shallow surface cavities and fractures in certain volcanic rocks. It forms during weathering of the rock by the gradual percolation of acidic solution containing dissolved minerals.

Turquoise has been valued by ancient Egyptians, Europeans and North American Frist Nations for thousands of years.

NATIVE ELEMENT MINERALS AND GEMS

Diamonds

Diamonds are probably more familiar and desirable than all other gems. Most engagement rings are diamond. The other gems we've studied are fairly complex minerals composed of various ions bonded together into ionic crystal lattices. Diamond, however, is a mineral uniquely composed of just one element, carbon, which is covalently bonded into a lattice.

Diamond is very hard, a maximum 10 on the Mohs scale. Nothing can scratch one except another diamond. The most outstanding characteristic of a diamond is its dispersion. No other gem breaks up light into the spectral colours of the rainbow like a diamond can. This gives diamonds incredible fire. Diamonds can also achieve amazing clarity, but only one fifth of diamonds mined have gem-quality clarity. Of that number, many have one or more visible inclusions, which can sometimes be hidden under the setting in jewelry. The finest diamonds are colourless but many diamonds have some colour, which can result from chemical impurities or from structural defects. Colour can detract or enhance a diamond's value. Richly pink and blue diamonds are priceless.

Diamonds are cut to maximize their brilliance (internal and external reflection), fire (spectral colours from light dispersion in the diamond) and scintillation (small flashes of light when a diamond or light source is moved).








Left is a rough diamond. Below left are various cut diamonds.

(Materialscientist:Wikipedia)







Diamond is composed of carbon atoms arranged in a tight regular cubic crystal arrangement called a face-centered cubic lattice, shown far right in the diagram below. This arrangement is the most tightly packed arrangement possible but it is not unique to diamond. Lead, aluminum, copper, gold and zinc also have this lattice arrangement.

(from Wikipedia: Cubic Crystal System

What makes diamond unique is the extremely strong covalent bonding between its carbon atoms. Thanks to this packed arrangement, diamond exhibits extreme hardness and thermal conductivity. Surprisingly, the chemical bonds in diamond, while very strong, are actually weaker than those in diamond's allotrope, graphite. In graphite (see a sample below top right), the same atoms are tightly bonded into sheets in a honeycomb-like two-dimensional lattice (bottom right in the image below). Although the bonds are stronger in graphite, graphite as a material is weaker than diamond. The sheets can slide over one another, weakening its overall structure. In diamond, the three-dimensional lattice bonds (bottom left) are inflexible. This means that diamond, though extremely hard is only moderately tough. A hammer blow can shatter it.

(Materialscientist; Wikipedia)

Diamond forms from graphite. Below is a theoretical phase diagram for diamond:

The hatched area is where both diamond and graphite phases coexist. (Ordinary air pressure is 1 Pa, or 0.001 Gpa, at bottom left, above. Room temperature is 20°C or 300K, between 0 and 1 bottom left, above).

Diamond is not chemically stable at ordinary room temperature. As the hatched area hints, diamond is metastable. However, itt will not decay under ordinary conditions because there is a high kinetic energy barrier it must overcome before the conversion to graphite will happen. As pressure increases, graphite converts into diamond. Over 1700°C (2000K), diamond begins to convert to graphite (the kinetic energy barrier is overcome). Its outer surfaces will blacken. But under high pressure, diamond is stable at temperatures of at least 3000°C.

What this means is that, despite what the "diamonds are forever" ad says, diamonds are technically not forever.

Diamonds are one of the very few gems that form in the mantle (a depth of around 150 to 190 km), rather than at the mantle/crust interface. These depths mean that diamonds form under thick stable plates. A long time at this pressure and heat allows diamond crystals to grow larger. Diamonds take a long time to form and a long time to reach the surface through natural plate movement - carbon isotope dating suggests that most diamonds are between 1 and 3.3 billion years old. Some diamonds are formed from inorganic carbon deep in the mantle. Others are formed from organic detritus (these diamonds would have had to have formed from primitive unicellular organisms) that is pushed deep under the surface through plate subduction.

Diamond Substitutes

Diamonds come with rich historic symbolism. Today they most often represent romantic love and commitment. Many men today present their beloved with an engagement ring when they propose. The ring custom originated in ancient Egypt. The ring symbolized a never-ending cycle as well as a gateway. It was later revived in Europe as a Posie ring, given to lovers during the late middle ages. It meant the promise of fidelity and love then as it does today. The first known diamond engagement ring was given to Anne of Burgundy in 1477. Soon the diamond ring became a very fashionable gift to loved ones, for those who could afford such luxury. Diamond rings didn't become common, however, until the 1930's. De Beer's miners discovered vast diamond finds in South Africa and launched a very successful marketing campaign to sell them all.

The lucrative and savvy diamond industry fueled the custom of the diamond engagement ring with powerful advertising, but like many lucrative industries, the diamond business has a dark side. Diamonds mined in war zones, sold to finance warlords, are coined "blood diamonds." Some brides-to-be, being equally savvy, now choose diamond substitutes for their engagement rings instead, and there are several.

Some diamond substitutes are better imitators than others.

Rhinestones are simply cut glass (SiO₂) backed by metallic foil. They are very inexpensive and they do sparkle. Cubic zirconia, a cubic crystalline form of zirconium dioxide (ZnO₂), is sparkly and has no imperfections, as shown here:

(Hadal; Wikipedia)

These manmade gems are also hard (around 8 on the Mohs scale) so they are quite durable. Different metal oxides can be added to them to create cubic zirconia in any rainbow colour. Coating the gem with diamond-like carbon makes them even harder and nearly as lustrous as diamond, achieving a refractive index of 2.18 compared to diamond's 2.42. Clear colourless zircons (ZnSiO₄) have as much lustre and fire as diamonds do and may be mistaken for diamonds by less experienced jewelers, but they are not quite as hard (7.5). White sapphires (Al₂O₃) are both brilliant and very hard (9.0) and make excellent diamond substitutes. Moissanite, a rarely found mineral in nature, is a man-made gem that is quickly growing in popularity. It is composed of large silicon carbide (SiC) crystals, which are covalently bonded together much like diamond. Moissanite rivals the brilliance, fire, lustre and hardness (9.5) of diamond, as shown below:

The two are very difficult to tell apart even by jewelers. It actually has a higher refractive index than diamond has (2.67 compared to 2.42 respectively). My daughter recently chose this gem for her engagement ring and I can attest that it is absolutely brilliant (and a great buy!).

Conclusion

Gems naturally draw us in with their powerful beauty. It's no wonder that, before scientific tools could tell us otherwise, people believed that gods, elves, sprites and other supernatural forces created them. Now we know that, as brilliant, colourful and otherworldly as gems may seem, they are formed just like other minerals through natural forces. The gods have been replaced by chemistry and physics.

Or have they? The fact that, through geological forces, some elements, which just happen to have the right set-up of electrons, come together in just the right ways under the right conditions to make minerals that just happen to play with light of wavelengths we can see doesn't seem to me to be entirely without some magic.

Gemstones Part 2 - Silicate Gems

Nearly all of Earth's crust (90%) is made up of minerals called silicates. It's no wonder that nature has fashioned brilliant gems from some of this abundant raw material.

All silicates are oxides, made up of oxygen (O) and silicon (Si) atoms, and they almost always come in the form of a complex tetrahedral shaped negative ion (SiO₄^4-). Each of the four oxygen ions in this complex ion can bond not only with the silicon ion in the middle of the complex but with other silicon ions as well, and this means that each tetrahedron can bond with another one. Single units, double units, chains, sheets and even intertwined three-dimensional arrangements are all possible, lending silicate gems a large variety of crystal shapes. This anion may also combine with various positive ions to make a huge variety of crystalline minerals. Silicon and oxygen can also bind simply into SiO₂, the mineral we know as sand or quartz.

QUARTZ

Many gems are based on a quartz structure, which is a uniquely twisting helical chain of tetrahedra. Each oxygen ion is shared between two tetrahedra, so this means quartz has an overall chemical formula of SiO₂, rather than SiO₄.

Quartz crystals are often twinned, sharing an oxygen ion between them, to make six-sided prism shapes. When these crystals are perfect, they exhibit an interesting and useful phenomenon called the Piezoelectric effect. This effect is put to great use in various electrical devices.

How a Quartz Watch Works

Quartz crystals, thanks to their unique structure, develop a charge separation between positive and negative ions. This creates an electric dipole. At equilibrium, all these dipoles are randomly oriented. When stressed however, the dipoles in quartz material organize themselves parallel to the direction of the stress, and when they do so, they generate an electric field (an electric potential across the crystal material). Conversely, when an electric field is applied, the crystal expands in the direction aligned with the field and contracts in the direction perpendicular to the field, as the dipoles adjust their alignment with the field. Voltage from a watch battery can set a small tuning fork shaped quartz crystal into oscillating. It is a standing wave oscillation with a resonant frequency related to the thickness of the crystal. This is what keeps time. Impurities in the crystal and poor crystal alignment are two main reasons why some cheap watches don't keep time very well.

(JJ Harrison; Wikipedia)

Quartz, clear and transparent in its pure form, is found in granite rocks formed as magma cools. When magma cools slowly, large crystals can form. These rocks are called pegmatite.. Pegmatities are very crystalline granite rocks, with large layered crystalline intrusions, usually quartz. Quartz crystals as large as several meters long have been found in this rock. Quartz crystals are also very common in sedimentary rocks like sandstone and shale. Like corundum (Al₂O₃), mentioned in the previous article, this mineral is hard and it resists weathering so it too is a part of river and beach sand.

If transitional metals are present during formation, various quartz-based gems can result.

QUARTZ GEMS

Gemstones such as citrine, rose quartz and amethyst are formed when trace crystal lattice substitutions with transition metals occur. The previous article in this series, "Gems - The Science of Their Colour" describes how this process works.

(de:Wela49: Wikipedia)

Citrine

Citrine (above right) is a rare yellow variation of quartz, which contains traces of iron {Fe³⁺).

Rose Quartz

Rose quartz contains trace amounts of titanium, iron and/or manganese:

(Rob Lavinsky/iRocks.com)

Amethyst

Amethyst, a globally abundant violet variety of quartz, is highly variable in intensity and hue. It is one of the most interesting gems because it can change colour and it usually comes in fascinating egg-like geodes. We'll explore how geodes are made in a moment.

Colour formation in amethyst is complex and not entirely understood. Researchers know that when quartz crystals form in the presence of iron (Fe³⁺) ions, some of these ions may substitute for silicon in the center of the tetrahedra. Some ion iron ions will also enter interstitial sites between these tetrahedra. The substitution scenario is well studied. Fe³⁺ alone doesn't account for amethyst's violet colour. Citrine (yellow) and greenish quartz also have this same iron substitution. Optical absorption studies show that amethyst has three main absorption peaks - all coming from iron in different valence states: Fe²⁺ in interstitial sites, substitution Fe³⁺ in some tetrahedra, and finally, substitution Fe⁴⁺ in other tetrahedra. Amethyst produces a complex absorption spectrum, so it is not easy to assign specific colours to the presence of one or more particular iron valences present. Greenish, colourless and smoky amethyst (as well as citrine as you will see) form right along with violet amethyst, and these variations most likely stem from the changing chemical and thermal nature of the precipitating solution that made the crystals. One colour-producing scenario seems to be gaining some traction - most researchers believe that gamma rays from radioactive material in the Earth cause some Fe³⁺ ions to lose another electron to make Fe⁴⁺ ions (change their valence). Fe⁴⁺ absorbs in the green-yellow range of the visible spectrum, allowing transmittance of colour in the blue-red range, contributing to amethyst's characteristic violet colour.

The substitution of other trace amounts of other transition metals may also occur, and they may give these gems reddish or bluish hues. Below is a carved amethyst portrait of Emperor Caracalia, from 212 AD.

(Marie-Lan Nguyen; Wikipedia)

Amethysts can fade over time in daylight but they can also be artificially darkened by (usually radium) irradiation. If a gem is heated it can turn yellow, orange or brown. Many citrines begin as amethysts that are later heated by further exposure to lava. Ametrine is a mixture of the two varieties, created when a temperature gradient is present during formation. Differential oxidation of iron ions occurs:

(de Wela49; Wikipedia)

Amethyst can be formed anywhere lava runs close to the surface. This is where amethyst geodes are made, and they can be up to several feet across. When lava flows over trees or gas bubbles created by convection, quartz crystals form in clusters along the inner surface of the bubble as silica-rich liquid seeps slowly through the porous volcanic rock and into the bubble:

(Saibling; Wikipedia)

BERYL GEMS

Emeralds

A mineral called beryl (Be₃Al₂(SiO₃)₅), a hexagonal crystal, may form along with quartz in pegmatite. Many gems are beryls, including the beautiful green emerald shown left. This gem-quality emerald crystal was found in a mine in Colombia. Trace impurities of chromium and sometimes vanadium give this mineral its intense green colour. The hexagonal crystals of beryl can range from small to very large - up to several meters in length in fact! Pure beryl is colourless but various impurities can make it blue, yellow or red as well as green.


(Mmlyncak; Wikipedia)

Aquamarines

The pale blue aquamarine, another beryl mineral, left - this one found in Pakistan - owes its colour to iron (Fe²⁺) impurities.











(Mmlyncak; Wikipedia)

Garnets

Most garnets are red but green, pale yellow, black and fiery orange ones can also be found. Their colour variation stems from variation in the garnet chemical formula, X₃Y₂(SiO₄)₃. X can be a divalent cation like Ca²⁺, Mg²⁺ or Fe²⁺. Y is a trivalent cation - Al³⁺, Fe³⁺ or Cr³⁺. Like the other gems we've looked at, these molecules arrange themselves in a regular crystal pattern. When calcium and aluminum ions occupy X and Y positions, you get Tsavorite, a spectacular green garnet, shown below as an uncut gem, giving any green emerald a run for its money.

(Rob Lavinsky/iRocks.com)

Moonstones

Moonstones, ((Na,K)AlSi₃O₈) grow as outgrowths of fine crystalline layers within pegmatite. These gems exhibit a mysterious moonlike shimmer known in the gem trade as adularescence. They are usually found in Sri Lanka (classical blue almost transparent ones), India (colourful ones) and in the European Alps. Their shimmer comes from their layered construction. It is formed when two intermingled minerals, orthoclase (KAlSi₃O₈) and albite (NaAlSi₃O₈), separate as the mineral cools into two thin alternating layers. Ambient light is refracted and scattered inside the stone. Moonstone cabochons play in the light of a shop window in England, below.

JADE

Jade is not a crystalline silicate gem. It is actually two different metamorphic rocks, made up of different silicate minerals. I include it here because it is a stone with rich history and symbolism, and because its formation is an interesting and not entirely understood story. Geologists know that it is made in subduction zones deep inside faults, where aqueous fluids under enormous pressure stream through open cracks and deposit minerals into massive veins. Blue and red luminescing jade forms first, then yellow/green jade and finally the fluid re-crystalizes into the familiar green jade we know. This conversion process is driven by the addition of new elements to the mix. Three different mineral-bearing fluids supply them. First, seawater, then aqueous solution squeezed out of rock in the vent, and finally fluid from the mantle magma itself all contribute sequentially to forming jade.

One kind of jade rock, shown below, called jadeite by mineralogists, is a pyroxene mineral (with the formula NaAlSi₂O₆)-rich rock, and is the rarer of the two:

The other variety is a rock called nephrite, rich with a mineral called nephrite with the hefty chemical formula, Ca₂(Mg, Fe)₅Si₈O₂(OH)₂. Craftsmen in British Columbia make jewelry and carvings from this kind of jade, which is almost always a shade of green. Jademine.com  tells us all about how jade in B.C. is mined (and they have some lovely items to buy online too). Nephrite jade is the famous ancient Chinese jade variety. This "imperial gem" was mined in China as early as 6000 BC.

Jadeite, available in emerald green, lavender, pink and orange, was imported into China beginning around 1800 (AD), as Chinese jade mines became depleted. Once the imperial family started using it to adorn their gravestones, this jade grew in value, some varieties of which are even more valuable than nephrite jade.  Both jades have fine veins, blemishes and streaks running through them. Specific patterns and an evenness in colour make specific specimens especially desirable. The Lost Laowai blog  provides a really interesting connoisseur's guide to Chinese jade.

ZIRCON

Zircon, technically called zirconium silicate (ZrSiO₄), is a common mineral found in all kinds of rock - igneous, sedimentary and metamorphic. These minerals can be slightly radioactive when they contain trace amounts of uranium in them. The most popular zircon gems are blue but they also come in dark red, green, violet, orange and brown. Some exceptional blue zircons, like the one below, can be very bright blue, an uncommon gem hue, thanks to internal dispersion of light and a high refractive index in these hard transparent gems.

Some zircons rival the sparkle of diamonds, though these gems, unlike diamonds, are quite brittle and must be carefully cared for.

Zircon gems are not the same thing as cubic zirconia. This manmade cubic crystal is explored along with other diamond substitutes in the next article in this Gems series, called "Non-silicate Gems."

OPALS

Opals are a uniquely non-crystalline silicate gemstone, almost all of which are found in Australia. They are mineraloids rather than true minerals because they do not have a crystalline structure. They are also hydrates - they contain anywhere from 6% to 21% water. This is the formula - SiO₂·nH₂O.

Although opals are amorphous (which means they have no regular structure), they can exhibit an internal structure composed of microscopic spheres of silica in a tightly packed lattice.

Spheres of silica form and settle out of silica-rich fluids into the Earth. If they are uniform in size at around 140 to 400 nm wide, they will make a gem-quality opal, because spheres in this size range diffract light in the visible range. To make an opal, the spheres settle into voids to a depth of around 40 metres at a rate that attains a 1 cm thickness in about 5 million years.

The lattice of spheres is what gives opals their many internal colours. Light passing through the spheres undergoes interference and diffraction and is separated into various rainbow-like colours. Tiny micro-fractures filled with secondary silica as well as thin lamellae (these require specific climate conditions - usually periodic wet/dry periods) formed during the gem's solidification give it its characteristic opalescence. Left is a sample of rough opal.

 

This polished opal gem, left, shows blue and green fire.





(CRPeters; Wikipedia)



TOPAZ

Topaz is a silicate gem containing aluminum and fluorine. It's chemical formula is Al₂SiO₄(F,OH)₂. Pure topaz consists of colourless transparent prismatic crystals. Like many gems, however, it may be tinted, by impurities, into wine, yellow, pale grey and reddish orange or blue/brown gems:

(Michelle Jo; Wikipedia)

Topaz has a high refractive index and it's very hard, 8 on the Mohs scale. The Mohs scale describes mineral hardness as scratch resistance, with talc being the softest mineral, 1, and diamond being the hardest mineral, 10.  These gems also display pleochroism. This means they can display two or more colours depending on the viewing angle.

Topaz gems may form along with other gem material, such as tourmaline (next) and beryl gems, in granitic pegmatite.

TOURMALINE GEMS

The last large group of silicate minerals is tourmaline, composed of aluminum boron silicate crystals. Unlike many of the silicate gems, corundum gems and diamonds, tourmaline gems don't have much folklore attached to them. In the last couple of centuries, however they have increased in popularity and value as gemstones. Tourmaline is a semi-precious gem that comes in a huge variety of colours, although at least 95% of tourmaline is found in nature as black Schorl. Multi-coloured tourmaline crystal formations often develop, creating unique design opportunities for jewelers. Unlike the colourful tourmalines, however, black Schorl is always opaque. It can form highly lustrous uniform crystals, however, creating collector-worthy pieces not unlike jet, and it is very popular as a mystical stone. It is believed to have powerful protective energy.

Structurally, all tourmalines have the same arrangement of atoms, an unusual triangular crystal lattice, but the chemical formula may contain up to 5 possible substitution points, which can be any of the transition metals. Tourmaline exhibits an unusual pyroelectric property. This means that when it is heated, a positive charge develops at one end of the crystal and a negative charge develops at the other end. Similarly, a charge can develop if pressure is applied to the ends of crystals. Tourmaline also displays piezoelectricity, like quartz does. Black Schorl doesn't show any pyroelectric properties, and it's only weakly piezoelectric. These qualities make non-gem quality (non-Schorl) tourmaline useful as an industrial material.

A pretty example of tourmaline is elbaite, Na(Li,Al)₃Al₆Si₆O₁₈(BO₃)₃(OH)₄. An example is the large pink crystal, called rubellite, in a quartz matrix from California, shown below left.

(Madereugeneandrew; Wikipedia)

Elbaite is often cut into gemstones. It can also come in blue (indicolite), green (verdilite) and in a highly sought after pink/green combination called watermelon tourmaline, above right.

Tourmaline gems are found along with other gem material, usually topaz and beryl gems, in granitic pegmatite.

Now that we've discovered an amazing variety of silicate gems, let's take a look at non-silicates, a much smaller group, which includes diamonds, next.

Gemstones Part 1 – The Science of Their Colour

BEAUTY AND MYSTERY

We've done lot's of scientific exploring so far. We've refined our reasoning, deduction and logic skills. We've slogged through a substantial and unavoidable amount of factual detail, and if you've read some of my other articles, and felt stumped and confused, I feel deep empathy. Know that I too have wanted to hurl my computer out the window many times (between you and me, some of the technical Wiki articles in particular are just awful torture for those of us who don't have the seemingly required Ph. D(s)). The road hasn't always been easy. So today let's load up our science with fun. Gems, to me, are not only fascinating but loaded with fun.

Gemstones, little glittering jewels sprinkled into the largely dull grey/brown matrix we call Earth's crust, beg us to let our imaginations free, to put our scientific tools aside for a moment, and allow their beauty to draw us in. The samples below are just the tip of the iceberg:

Until recently, gems have been an utter mystery to humans. We didn't know where they come from, how they are made, and why they are so stunningly beautiful. For centuries, gemstones have served as tiny vessels allowing mystery and beauty to condense into various restorative and magical powers ready to bestow on us a passageway to magic simply by looking at them.

Rich folklore developed around many gemstones and I urge you to browse some of these fascinating stories. Ellen Steiber provides us with an excellent introduction to gemstone meanings and symbolisms. Mysticalencounter provides a primer and an extensive list of gemstone mystical powers. If you consider our contemporary lore around diamonds and birthstones, you'll recognize some of the powerful symbolic power that still pervades the world of gems today.

Here are a variety of websites perfect for ogling beautiful gems (they have great photos):

Gemselect. Click on each gem to see even more gem photos.

The Mineral and Gemstone Kingdom photo gallery

Alpine Gems, the "Canadian Gemstone" website

Production and Photographs of American Gems 

Minerals By Name (An extensive alphabetic list with photographs)

Gyspy Gemstones

Birthstones

John Betts Fine Minerals

Try out a new age crystal shop, pick up and turn over samples in your hands, marvel at all of the intoxicating colours, textures and how they play with the light. Where do they come from? How are they made? And what are they made of?

Browse, relax, set your scientific toolkit aside for a few moments and marvel at each gem's natural magic.

THE SCIENCE

Now that we are in a relaxed place and our chakras are in order, let's ease our way into the science.

Gems, Minerals and Rocks

Gems, minerals, rock - this is the science of geology. Gems and minerals are actually the same thing. But only some minerals earn the name "gemstone." Minerals are naturally occurring substances that are stable solids with specific chemical formulas and ordered atomic structures, and there are a lot of them, about 4300 here on Earth. Minerologists (geologists who focus their study on minerals) utilize a variety of properties such as colour, luster, crystal hardness and transparency in order to identify and characterize them all. What makes a mineral a gemstone is part common sense (hardness, scratch resistance) and part subjective quality- it must have clarity, colour and "fire" - the ability to split light into the colours of the rainbow. The technical word for fire is dispersion, but fire is so much more evocative of the effect. Brilliance, luster and scintillation are other words gemologists use. We will explore these qualities soon. Ultimately what makes a mineral a gem is its beauty.

A mineral is not a rock. Minerals have specific chemical formulas. Rock, however, is a less specific term for all the crust material on Earth. A rock can be an aggregate of different minerals or it may not be made of minerals at all.

To make things even trickier, gems are often minerals but not always. Lapis lazuli is a rock gem that has been mined in Afghanistan since 3000 BC, while amber and jet are organic materials.

Below left is an amazing polished specimen of lapis lazuli.

(fr:User:Luna04; Wikipedia) and (Anders L. Damgaard - www.amber-inclusions.dk;Wikipedia)

Amber, right, is fossilized tree resin, and it has been valued as a gem for over 10,000 years. An ant is perfectly preserved inside this specimen.

Jet, perfectly black, can be polished to a glassy shine. It is composed of carbon
from decaying wood squeezed under enormous pressure. Jet jewelry dates
back to 17,000 BC.

How Gems Are Formed

The Earth's crust is made of several plates that float upon a liquid magma mantle. Where the two layers meet is a zone of enormous pressure and extreme temperatures. The lower surface of the crust, where it meets the mantle churning beneath it, contains many fractures and fluid-filled cavities. These fluids come from mineral-rich magma. As it rises toward the surface during convection, the pressure exerted on it decreases enough to allow gases to escape. These gases can later condense inside rock cavities, creating mineral-rich fluids. Crystals can start to form in the fluid. The cooling of the magma must be very gradual in order for gems to form. Gem-quality crystal formation requires intense heat and pressure, with very gradual cooling. If the magma is too close to the surface, it will cool rapidly and the crystals will be too small. Granite rock will form. It contains small crystal grains of quartz and other minerals that could be the building blocks of gems if they were larger. If the cooling is gradual enough, pegmatite will form instead of granite. It is made of the same minerals but it is much more coarsely grained, with pockets inside it where gemstones with large well-formed crystal structures can form. It is the intense pressure that forces atoms into various tight crystal lattices associated with many gems. This heat and pressure also contributes to the translucency or transparency of many gems. The International Gem Society provides an in-depth explanation of how and where various gems form.

One Formula > Several Gemstones: Corundum Gems As an Example

Many gems (there are over 200 recognized types) come in several different varieties, meaning that one mineral (a single chemical formula) can account for several, often very distinct, gems. For example, sapphires and rubies are both corundum, an aluminum oxide (Al₂O₃) mineral. Corundum is the hardest of any natural mineral besides diamond. It is often mixed in with grains of beach and river sand (silicon dioxide). It is used as sandpaper grit.

The atoms in corundum, like many but not all gemstones, are organized into tightly fitted together lattices called crystals. Each crystal below is a sample of corundum:

(Ra'ike; Wikipedia)

None of these samples look anything like a ruby or a sapphire. Pure corundum is colourless. It is the inclusion of other elements, usually metals, which give it a range of colours: pink, red, yellow, blue, violet and green.

Rubies and sapphires (and all corundum minerals) have exactly the same crystal structure:

(NIMSoffice; Wikipedia)

What makes some corundum minerals colourful gems is the trace substitution of other metal ions for aluminum ions. Chromium, titanium and iron can all substitute for Al³⁺, and this substitution is part of the secret of why gems have so many brilliant colours.

left: (Montanbw; Wikipedia) and (Humanfeather:Wikipedia)

A sapphire, above left, has the same aluminum oxide (Al₂O₃) mineral structure as the corundum examples. But here, trace amounts of two metal ions - iron (Fe²⁺ or Fe³⁺) and titanium (Ti⁴⁺), substitute for Al³⁺ ions in aluminum oxide.

A ruby on the other hand, below left, owes its intense red colour to chromium. Like the sapphire, a few Al³⁺ ions are replaced by other metal ions, in this case chromium (Cr³⁺).

The Secret of Colour

Our ruby and sapphire examples show us that substitutions with certain metal ions in the crystal structure of corundum give us two very different gems. Aluminum, chromium, titanium and iron are all transition metals.

Transition metals, below, are located in the d-block in the periodic table:

Colours and Our Eyes

Before we explore how metal substitutions create brilliant gem colours, let's familiarize ourselves with how our eyes perceive them. The colour of objects is a very complex science in and of itself. For our purposes, I will simplify by saying that the colour of an object tends to be complimentary (opposite) to the colour that its molecules absorb.

This is a colour wheel. Colours on opposite sides of the wheel are complimentary to each other.

(Sakurambo; Wikipedia)

For example, green leaves reflect green light. The green chlorophyll in them absorbs light at both the red end and the blue end of the visible light (sunlight) spectrum, but not green light. The light reflected from the leaf is missing red and blue and therefore it looks green as a result.

How our eyes and brains perceive the colours of various objects is actually very complex, depending on how light reflects or scatters off the object (physical factors), and how our brains interpret the light based on ambient lighting, viewing angle, etc. (an effect called colour constancy).

Some gems even emit light, a phenomenon responsible for the brilliant red intensity of some rubies, as we will see in a moment.

Valence and Colour

Atoms have various energy levels available to their electrons. These are called shells. Atoms always fill up their innermost shells first and move outward from there. The outermost shell contains the electrons that are available for chemical bonding. These are called valence electrons.

The valence number of an element tells you how many covalent bonds that element can form with other atoms. The arrangement of electrons in an atom becomes most stable when the valence shell is either filled or emptied. This is one reason why atoms want to bond with each other - to share electrons and fill their valence shells.

We already saw that iron ions can exist in two different states - Fe²⁺ and Fe³⁺. These are called oxidation states. Valence number and oxidation state are the same thing. Some elements have only one valence state, while others like iron have more than one. All the transition metals in the grid above have partially filled valence shells.

Ions containing transition metals, metals with partially filled d valence shells, tend to be coloured.

For example, copper (Cu²⁺) in solution is light blue. When (an anhydrous - no water - white powder) cupper (II) sulfate is dissolved in water, each copper ion forms an octagonal complex as six water molecules bond with it, along with 2 negative sulfate ions. This happens because water ionizes in solution into a hydroxide (OH^-) ion and a hydronium (H₃0⁺) ion, as shown here:

Positively charged copper ions will bond with negatively charged hydroxide ions and negatively charged sulfate ions to form a complex. Other transition metal ions in aqueous (water) solution also form various kinds of octagonal arrangements. And when they do, they tend to create energy gaps, which vary depending on the transition metal ion, its oxidation state and the nature of the ligands (the surrounding ions or molecules).

Energy gaps are the key to colour formation. The copper ion has 9 electrons in its outermost (valence) energy shell, all occupying 5 3d orbitals. Electrons in two of the 3d orbitals align with the negative ions as they approach but they also repel them, being negatively charged. This arrangement (of being pushed together) increases the energy of these two 3d orbital electrons. Now, the octagonal complex can absorb a light photon that has an energy equivalent to the energy difference between the electrons in the 2 higher energy 3d orbitals and the remaining 3 regular energy 3d orbital electrons. This phenomenon is called crystal field splitting. The particular energy in the case of this copper/water complex is in the orange region of the spectrum, so orange light is absorbed.

The promotion of each of the three electrons absorbs the amount of energy corresponding to a photon of orange light. The light reflected from the complex (missing orange photons) looks blue as a result, so the Cu²⁺ solution looks blue to us.

(This is also why non-anhydrous cupper sulfate crystals look blue. These crystals are in a hydrate form, Cu(II) SO₄·5H₂O. Five water molecules are bound to the copper sulfate.)

This kind of mechanism is especially prevalent in transition metal complexes, which are formed when various gemstones crystallize from aqueous solutions. In order to have colour, the energy gap must be between 1.8 and 3.1 electron volts (eV), because this energy corresponds to the energy of light photons in the visible range, with wavelengths between 400 to 700 nm (nanometers). This range represens very low energy transitions, so they require very closely spaced electron shells, some of which contain electrons and some of which are empty. The partly filled d orbitals of transition metals fit the bill. In most compounds, the energy required for the promotion of an electron requires more than 6 eV. Photons need to be at least in the ultraviolet range to have this much energy. Such compounds are absorbing these UV photons and reflecting light but we can't see it.

Vanadium is another pretty transition metal ion. Dissolved in water, it can achieve one of four different valence states (depending on the solution's pH) of +2 (lilac), +3 (green), +4 (blue) or +5 (yellow), as shown from left to right below:

The cupper and vanadium examples show us how transition metal ions in a crystal can create energy gaps in gemstones that result in vivid colours. Both the sapphire and the ruby exhibit this phenomenon of crystal field splitting to achieve blue and red, respectively. This is not the whole story, however. Colours in gems can be intensified further through various additional mechanisms as well. Again, we will use the sapphire and the ruby as our examples.

Gems Boost Colour Further

Blue sapphires owe their intense blue colour to fairly complex chemistry. A blue boost comes from a transfer of an electron from one metal to another, usually iron (Fe²⁺ or Fe³⁺) to titanium (Ti⁴⁺), both of which are impurities that substitute for Al³⁺ ions in the aluminum oxide crystal lattice. This electron transfer creates localized pockets of charge imbalance. It causes a change in the valence states of both iron and titanium. When the iron and titanium ions change valence states, electromagnetic energy is absorbed, specifically yellow light. When an object absorbs yellow light, it looks blue, yellow's complimentary colour, because yellow is missing from the wavelengths reflected from it. It only takes 0.01% titanium and iron impurities in corundum to make the intense blue in a gem-quality blue sapphire.

Like the sapphire, in a ruby, Al³⁺ ions are replaced by other metal ions, in this case chromium (Cr³⁺). What is unique here is that each Cr³⁺ ion is surrounded by six oxygen ions (O^2-), creating a crystal arrangement that can achieve two light tricks at the same time, making rubies appear especially brilliant to our appreciative eyes. First, yellow-green light is absorbed by it (through the mechanism of crystal field splitting similar to that of the sapphire). Second, this absorbed light is then re-emitted at a different and specific wavelength as red luminescence. The chromium ions, by absorbing these photons, gain enough energy to emit new photons with a wavelength of exactly 694 nm, pure red light in other words.

Laser technology makes use of this special situation. To make a ruby laser, a tube-shaped artificial ruby is capped on each end by mirrors, one totally silvered and the other half silvered. When light is shone on the tube, chromium atoms in the ruby become excited. Just like above, some of them emit red photons of a specific wavelength. Some of these photons run parallel with the tube so they bounce off the mirrors at each end, stimulating other atoms to emit even more red photons. You end up with a single-phase monochromatic red light beam that eventually leaves through the half silvered mirror, a red laser.

Other Optical Behaviours In Gems

Besides the ability to stun us with vibrant colours, often by using more than one chemical mechanism, gems are ingenious with light. Again, thanks to their tight crystalline chemical structure, they are able to take ambient light and play with it like no other natural material can. Gems are an amusement ride for photons. This light-play is called scintillation.

Refraction

Gems are not flat opaque objects, so not all light is simply absorbed or reflected off their surfaces. Because they often exhibit at least some transparency, much of the light striking the gem will refract into it. This means that the phase velocity of the light traveling from air into the gem at an oblique angle is changed. When that happens, it cause the light beam to change direction. Each gem has its own specific index of refraction. Light entering a gem at its critical angle will achieve total internal reflection. If light intersects with the gem within its critical angle, that facet will act like a mirror and it reflect right off of it.

Gems take light in at every facet (surface) so there is a great deal of refraction going on. Gems with high refractive indexes exhibit more refraction, and because of increased refraction they exhibit greater brilliance and lustre.  Diamond, for example, has a refractive index of 2.4, while most glasses have an index of around 1.5. This is why glass cut the same as a diamond won't have nearly as much brilliance. Specific gems are cut at particular facet angles to maximize refraction.

Dispersion

Gems are also prismatic crystals so they can also disperse light into rainbow-like colours. This effect is also due to refraction of white light into its component colours. You can often see this effect on the crown facet (top surface) of gemstones, where you can observe flashes of colours. This is part of the gem's fire. It is most easily observed in light coloured gems, especially diamonds.

Double Refraction

Double refraction also occurs in almost all gemstones, in which incoming light is divided into two rays as it enters the crystal, by two different molecules inside it. You will sometimes see a visual doubling of facets when this occurs. The effect is most notable in zircons:

(Oneworldgemstone; Gemologyproject.com; Wikipedia)

Reflection

Reflection off the hard polished surface of a gem contributes to its external lustre. Light also enters the stone, reflects internally, treating the internal surfaces of the gem like mirrors, and then returns to the eye. This is the gem's internal lustre. Both contribute greatly to the brilliance of the gem.

The tight crystalline structures of gems multiply their beauty by enhancing the various ways that light can play in and off of them. Allaboutgemstones explains all of these plays of light in gems very well.

In the next article we will explore an enormous class of gems called silicate gems. This group includes some of the most beautiful gems available.