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 (SiO44-). 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
(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:
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.
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