Something Precious, Gemstones
Something Precious, Gemstones
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| The Hope Diamond. |
Mystery and romance follow famous gems. Consider the stone now known as the Hope Diamond, recognized by name the world over (figure above). No one knows who first dug it out of the ground (See Where Do Diamonds Come From?). Was it mined in the 1600s, or was it stolen off an ancient religious monument? What we do know is that in the 1600s, a French trader named Jean Baptiste Tavernier obtained a large (112.5 carats, where 1 carat 200 milligrams), rare blue diamond in India, perhaps from a Hindu statue, and carried it back to France. King Louis XIV bought the diamond and had it fashioned into a jewel of 68 carats. This jewel vanished in 1762 during a burglary. Perhaps it was lost forever perhaps not. In 1830, a 44.5-carat blue diamond mysteriously appeared on the jewel market for sale.
Henry Hope, a British banker, purchased the stone, which then became known as the Hope Diamond. It changed hands several times until 1958, when a famous New York jeweller named Harry Winston donated it to the Smithsonian Institution in Washington, DC, where it now sits behind bulletproof glass in a heavily guarded display.
What makes stones such as the Hope Diamond so special that people risk life and fortune to obtain them? What is the difference between a gemstone, a gem, and any other mineral? A gemstone is a mineral that has special value because it is rare and people consider it beautiful. A gem, or jewel, is a finished stone ready to be set in jewellery. Jewellers distinguish between precious stones (such as diamond, ruby, sapphire, and emerald), which are particularly rare and expensive, and semiprecious stones (such as topaz, tourmaline, aquamarine, and garnet), which are less rare and less expensive. All the stones mentioned so far are transparent crystals, though most have some colour. The category
of semiprecious stones also includes opaque or translucent minerals such as lapis, malachite, and opal.
In everyday language, pearls and amber may also be considered gemstones. Unlike diamonds and garnets, which form inorganically in rocks, pearls form in living oysters when the oyster extracts calcium and carbonate ions from water and precipitates them around an impurity, such as a sand grain, embedded in its body. Thus, pearls are a result of biomineralization. Most pearls used in jewellery today are “cultured” pearls, made by artificially introducing round sand grains into oysters in order to stimulate pearl production. Amber is also formed by organic processes it consists of fossilized tree sap. But because amber consists of organic compounds that are not arranged in a crystal structure, it does not meet the definition of a mineral.
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| Cutting gemstones. |
In some cases, gemstones are merely pretty and rare versions of more common minerals. For example, ruby is a special version of the common mineral corundum, and emerald is a special version of the common mineral beryl (figure above a). As for the beauty of a gemstone, this quality lies basically in its colour and, in the case of transparent gems, its “fire” the way the mineral bends and internally reflects the light passing through it, and disperses the light into a spectrum. Fire makes a diamond sparkle more than a similarly cut piece of glass.
Gemstones form in many ways. Some solidify from a melt, some form by diffusion, some precipitate out of a water solution in cracks, and some are a consequence of the chemical interaction of rock with water near the Earth’s surface. Many gems come from pegmatites, particularly coarse-grained rocks formed by the solidification of steamy melt.
Most gems used in jewellery are “cut” stones, meaning that they are not raw crystals right from the ground, but rather have been faceted. The smooth facets on a gem are ground and polished surfaces made with a faceting machine (figure above b). Facets are not the natural crystal faces of the mineral, nor are they cleavage planes, though gem cutters sometimes make the facets parallel to cleavage directions and will try to break a large gemstone into smaller pieces by splitting it on a cleavage plane. A faceting machine consists of a doping arm, a device that holds a stone in a specific orientation, and a lap, a rotating disk covered with a wet paste of grinding powder and water. The gem cutter fixes a gemstone to the end of the doping arm and positions the arm so that it holds the stone against the moving lap. The movement of the lap grinds a facet. When the facet is complete, the gem cutter rotates the arm by a specific angle, lowers the stone, and grinds another facet. The geometry of the facets defines the cut of the stone. Different cuts have names, such as “brilliant,” “French,” “star,” and “pear.” Grinding facets is a lot of work a typical engagement-ring diamond with a brilliant cut has 57 facets (figure above c).
Where Do Diamonds Come From?
Diamonds consist of carbon, which typically accumulates only at or near Earth’s surface. Experiments demonstrate that the pressures needed to form diamond are so extreme that, in nature, they generally occur only at depths of around 150 km below the Earth’s surface. Nowadays, engineers can duplicate these conditions in the laboratory, so corporations manufacture several tons of synthetic diamonds a year.
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| Diamond occurrences. |
How does carbon get down to depths of 150 km? Geologists speculate that subduction or collision carries carbon- containing rocks and sediments down to the depth where it transforms into diamond beneath continents. But if diamonds form at great depth, how do they return to the surface? Some diamonds rise when rifting cracks the continental crust and causes a small part of the underlying mantle to melt. Magma generated during this process rises to the surface, bringing the diamonds with it. Near the surface, the magma solidifies to form an igneous rock called kimberlite, named for Kimberley, South Africa. Diamonds brought up with the magma are embedded as crystals in solid kimberlite (figure above). Much of the world’s diamond supply comes from mines in this rock. But some sources occur in deposits of sediment formed from the breakdown and erosion of kimberlite that had been exposed at the surface. Rivers and glaciers may transport diamond- bearing sediments far from their original bedrock source.
Not all natural diamonds are valuable; the value depends on colour and clarity. Diamonds that contain imperfections (cracks, or specks of other material), or are dark gray in colour, are not used for jewellery. These stones, called industrial diamonds, are used as abrasives. Gem-quality diamonds come in a range of sizes. Jewellers measure the size of these gems in carats,
where one carat equals 200 milligrams (0.2 gram). In English units of measurement, one ounce equals 142 carats. The largest diamond ever found, a stone called the Cullinan Diamond, was discovered in South Africa in 1905, and weighed 3,106 carats (621 grams) before being cut. By comparison, the diamond on a typical engagement ring weighs less than one carat. Gem-quality diamonds are actually more common than you might expect suppliers stockpile the stones in order to avoid flooding the market and lowering the price.
Credits: Stephen Marshak (Essentials of Geology)
