Spinels are a class of minerals with the chemical formula A2+B23+O42-. 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:
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, 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.
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:
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
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:
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 (S2-) 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:
Phosphates, in which a negative phosphate ion (PO43-) 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:
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 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 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.
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.
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.
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:
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!).
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.