The stones

Diamond is carbon crystallized in the cubic system and the hardest natural material, 10 on the Mohs scale. GIA gives its refractive index as 2.42 and its specific gravity as 3.52, plus or minus 0.01. Most gem diamonds fall in the colorless to light yellow or brown range that GIA grades from D to Z. Stones with deeper color, or with other hues, are fancy colors, and their value generally rises with the strength and purity of the color rather than falling with it.

Nitrogen causes yellow. Diamonds holding isolated nitrogen atoms, the type Ib stones, include the intense yellows the trade calls canary. Boron makes diamonds blue, and fewer than one boron atom per million carbon atoms is enough to do it. Pink to red color in natural diamonds usually comes from a broad absorption band near 550 nm that is linked to plastic deformation of the lattice; the same band is common in brown stones, and the defect behind it is still poorly understood. Green usually comes from GR1, a vacancy created by radiation damage. Laboratory irradiation produces the same defect, so natural green is among the hardest colors to prove.

Scientists sort diamonds into types by the nitrogen and boron atoms in their lattice, usually measured with infrared spectroscopy. The system grew from work by Robertson and colleagues in 1934 and 1936, who divided diamonds by how they transmit ultraviolet and infrared light; Kaiser and Bond linked yellow color to nitrogen in 1959.

Type Ia diamonds, more than 95% of natural stones, hold nitrogen in aggregates. Type Ib diamonds, a rare few, contain isolated nitrogen atoms and are almost always brown, yellow or orange. Type IIa diamonds have no easily measurable nitrogen or boron; the Golconda region of India is their historic source. Type IIb diamonds contain boron, which gives them their blue or gray color and their electrical conductivity; most come from India and the Cullinan mine in South Africa. Laboratories also use type as an early clue when screening for treatment or laboratory growth.

Black diamonds include carbonado, an opaque, porous, polycrystalline form found only in Brazil and the Central African Republic. Not one carbonado has been reported from kimberlite. Its density runs from 2.8–3.45, most of it near 3.05, against 3.52 for gem diamond, and its origin is unresolved, with proposals ranging from crustal growth to an extraterrestrial source. The largest known piece, the Sergio, was recovered in 1905 and is 61 ct heavier than the 3,106 ct Cullinan rough.

Corundum is aluminum oxide, Al₂O₃, rated 9 on the Mohs scale, with a refractive index of 1.762–1.770 and a specific gravity of 4.00. Pure corundum is colorless, and trace elements produce its colors. Stones colored red by chromium are ruby, in hues from orangy red to purplish red; the more chromium, the stronger the red. Chromium also causes fluorescence. Rubies formed in marble contain little iron, so their red fluorescence survives and shows even in sunlight.

Blue sapphire owes its color to iron and titanium, a few hundredths of a percent of each, and more iron means a darker blue. Padparadscha is a rare pinkish orange sapphire named after the lotus flower. Corundum of every other hue, including violet, green, yellow, orange, pink and purple, along with parti-colored stones, is sold as fancy sapphire. Color-change sapphires most often shift from blue in daylight or fluorescent light to purple under incandescent light.

Star rubies and star sapphires show asterism, usually a six-rayed star across the curved surface of a cabochon. It arises from white light reflecting off numerous tiny oriented needle-like inclusions; in sapphire these are often the intersecting rutile needles gemologists call silk.

Beryl is a beryllium aluminum silicate, Be₃Al₂Si₆O₁₈, that grows as hexagonal prisms and rates 7.5–8 on the Mohs scale. GIA gives the species a refractive index of 1.577–1.583 and a specific gravity of 2.72. Pure beryl is colorless and is called goshenite, and small amounts of other elements produce the colored varieties.

Emerald is the green to bluish green variety. Parts of the trade give the name to any green beryl colored by chromium, while most gemologists, laboratories and dealers call a stone green beryl when its color is too light for emerald. GIA settles the question against lab-graded comparison stones. Emeralds are typically included, and the trade accepts visible inclusions in emerald more readily than in most gems.

Aquamarine is greenish blue and light in tone. Morganite, pink to orange-pink, takes its color from traces of manganese and reads a little higher, 1.583–1.590 and 2.80–2.91. Red beryl owes its color to Mn³⁺. It was first reported from Utah's Thomas Range in 1905 and found on the eastern flank of the Wah Wah Mountains in the late 1950s, still the source of nearly all gem material. Its early name bixbite never came into wide use.

Chrysoberyl is beryllium aluminum oxide, BeAl₂O₄, an orthorhombic mineral rated 8.5 on the Mohs scale, which makes it the third-hardest gem in common use after diamond and corundum. Most chrysoberyl is yellow, greenish yellow or brown, but two varieties command high prices.

Alexandrite is the color-change variety: bluish green in daylight, purplish red under incandescent light. GIA gives it a refractive index of 1.746–1.755 and a specific gravity of 3.73. The change does not come from pleochroism, although alexandrite is strongly pleochroic, but from the way the mineral absorbs light, so the balance tips one way under daylight and the other under lamplight. The variety was first found in Russia's Ural Mountains in the 1830s and named for the future Tsar Alexander II, who freed Russia's serfs and was assassinated in 1881. Sri Lanka, East Africa and Brazil are now its main sources.

Cat's-eye chrysoberyl, also called cymophane, shows chatoyancy, a single bright band of light across a domed cabochon. Its eye is sharper than the softer, more diffused eye of cat's-eye tourmaline. Cat's-eye alexandrites, which combine both effects in one stone, are exceptionally rare.

Garnet is a group of cubic silicate minerals that share one crystal structure but hold different metals in it. More than twenty species are known; five matter commercially, and many gem garnets are chemical mixtures of two or more of them. GIA gives the group a refractive index of 1.714–1.888, a specific gravity of 3.47–4.15 and a hardness of 6.5–7.5, with no double refraction.

Pyrope, Mg₃Al₂(SiO₄)₃, and almandine, Fe₃Al₂(SiO₄)₃, supply most red garnet. Rhodolite, (Mg,Fe)₃Al₂(SiO₄)₃, is the purple-red blend of the two. Spessartine, Mn₃Al₂(SiO₄)₃, is the orange species.

Grossular, Ca₃Al₂(SiO₄)₃, occurs in nearly every color except blue and reads 1.731–1.760 with a specific gravity of 3.40–3.78. Its green variety tsavorite is narrower, 1.739–1.744 and 3.57–3.65, and is colored mainly by vanadium with some chromium. Hessonite is orange to cinnamon brown grossular. Andradite, Ca₃Fe₂(SiO₄)₃, yields demantoid, a chromium-green garnet reading 1.880–1.889 with a specific gravity of 3.80–3.88 and a dispersion of 0.057, higher than diamond's 0.044. Its radiating horsetail inclusions are fibers of serpentine growing from a tiny chromite crystal.

Color-change garnets shift hue between daylight and incandescent light, as alexandrite does.

Tourmaline is a group of closely related boron silicates with trigonal crystals, often long prisms with a three-sided cross section, which no other common mineral forms. The group shares silicon, aluminum and boron and takes in sodium, lithium, calcium, magnesium, manganese, iron, chromium, vanadium, fluorine and sometimes copper, which is why it occurs in more colors than almost any other gem. Hardness is 7–7.5, refractive index 1.624–1.644 and specific gravity about 3.06. Most gem material is lithium-rich elbaite, Na(Li₁.₅,Al₁.₅)Al₆Si₆O₁₈(BO₃)₃(OH)₄. Tourmaline is pyroelectric, taking an electric charge when heated, and piezoelectric when squeezed.

Trade names follow color. Iron, and possibly titanium, are generally held to cause green and blue; manganese produces reds and pinks. Rubellite is pink, red, purplish red, orangy red or brownish red. Indicolite is dark violetish blue, blue or greenish blue. Paraíba tourmaline, from the Brazilian state of that name, is an intense violetish blue, greenish blue or blue colored by copper. Chrome tourmaline, from Tanzania, is intense green but is colored mostly by vanadium, not by the chromium its name suggests.

Many crystals change color along their length or from core to rim. Watermelon tourmaline is pink in the center and green around the outside, and is usually sliced to show the pattern.

Spinel is magnesium aluminum oxide, MgAl₂O₄, with a cubic structure, a hardness of 8, a refractive index of 1.718 and a specific gravity of 3.60. Because cubic crystals are singly refractive, spinel shows the same color in every direction, unlike ruby and sapphire, which can look different along different crystal axes. It runs from orange through red, pink and every shade of purple, blue and violet to bluish green.

For centuries red spinel was not distinguished from ruby, and many of the famous historical "rubies" are spinels. Two of the best-known in the British crown jewels, the Black Prince's Ruby and the Timur Ruby, are red spinel. The mineralogist Jean Baptiste Louis Rome de Lisle separated spinel from ruby as a distinct mineral.

Chromium produces the intense reds and pinks, and the more chromium a stone holds the stronger its red. Orange and purple stones owe their color to a mixture of iron and chromium, violet to blue spinel to iron alone, and the saturated stones sold as cobalt blue spinel to traces of cobalt. Myanmar and Sri Lanka are long-established sources.

Quartz is silicon dioxide, SiO₂, with a hardness of 7, a refractive index of 1.544–1.553 and a specific gravity of 2.66. Its gems come in two textures: single crystals cut into transparent faceted stones, and chalcedony, a compact mass of microscopic quartz fibers.

Amethyst, the purple variety, runs from light lilac to deep royal purple and from brownish to vivid, and commonly shows angular zones of darker and lighter color. Citrine is yellow to orangy red, colored by a trace of iron. Natural citrine is rare, and most citrine on the market is heat-treated amethyst; whole amethyst geodes from Brazil are often heated to become citrine. In Bolivia the two colors occur in one crystal, and those stones are sold as ametrine. Smoky quartz is light brown to black and rose quartz a light to medium pink; both share the properties of the species.

Chalcedony varieties are named by color and pattern. Agate is banded, and onyx has straight parallel bands; most black onyx in commerce is dyed. Carnelian is orange to red, chrysoprase apple green and jasper the opaque, often patterned member of the family. Much commercial chalcedony is dyed or heated.

Opal is hydrated silica, SiO₂·nH₂O, holding up to about 20% water in its silica structure. It has no crystal structure and is classed as a mineraloid. GIA gives a hardness of 5–6.5, a refractive index of 1.37–1.47 and a specific gravity of 2.15 with a wide tolerance. Water loss can leave some opal prone to fine surface cracking, called crazing.

Precious opal shows play-of-color, flashes of spectral color that shift as the stone moves. The cause is diffraction: opal is built of sub-microscopic silica spheres stacked in a regular grid, and sphere size decides which colors appear, with spheres about 0.1 micrometre across giving violet and about 0.2 micrometre giving red. Common opal lacks the ordered stacking and shows no play-of-color.

Trade names mostly describe body color and host rock. Black opal, the rarest of the main types, has a dark body tone that makes the colors stand out. White or light opal has a pale body color and is more common. Boulder opal is cut with its ironstone matrix attached, which becomes part of the finished gem. Fire opal is transparent to translucent with a yellow, orange or red body color, with or without play-of-color, and is also called Mexican opal. Australia has been the primary source since its mines began producing commercially in the 1890s.

Jade is a trade name. GIA applies it to nephrite, jadeite and, under certain conditions, green omphacite, all of them dense mats of interlocking microscopic crystals. In 1863 the French mineralogist Alexis Damour analyzed bright green jades from Burma, found them different from the amphibole jade long known in China, and named the mineral jadeite.

Jadeite is a sodium aluminum pyroxene, NaAlSi₂O₆, with a hardness of 6.5–7, a refractive index of 1.666–1.680 and a specific gravity of 3.34. To count as jadeite jade, jadeitic pyroxene should make up 90%–95% of the rock. Chromium produces the near-transparent vivid green called imperial jade, the most valuable color. Myanmar remains the primary source of top-grade material, worked in the Hpakan district of Kachin State; jadeite also occurs in Guatemala, Japan, Russia and California.

Nephrite lies in the actinolite-tremolite series of amphiboles. It is softer at 6–6.5 and lighter at 2.95, but its felted fibrous texture makes it supremely tough among rocks. Its historic Chinese source was south of Hotan, in the Kunlun mountains. CIBJO gives the correct trade names as jadeite or jadeite-jade and nephrite or nephrite-jade, and requires disclosure of the polymer impregnation and dyeing commonly applied to jadeite.

Tanzanite is the violet blue to violet purple variety of zoisite, Ca₂Al₃(SiO₄)₃(OH), found in the Merelani hills of northern Tanzania in the 1960s and still commercially mined only there. It is strongly pleochroic, and most stones are heated, which removes or reduces the yellow green or brownish pleochroic color and leaves more blue and violet.

Topaz, Al₂SiO₄(F,OH)₂, is hard at 8 but has perfect cleavage in one direction. Its color is allochromatic: chromium accounts for natural pink, red and violet to purple, while defects in the crystal structure account for yellow, brown and blue. Most blue topaz is colorless material irradiated with electrons and then heated. Imperial topaz, a medium reddish orange to orange-red, comes commercially from one area, Ouro Preto in Brazil.

Zircon, ZrSiO₄, is a natural mineral, unrelated to synthetic cubic zirconia. Uranium and thorium damage the lattice over geologic time, giving low zircon much lower optical and physical properties; heating to high temperatures repairs some of that damage, and nearly all jewelry zircon is of the high type.

Peridot is gem olivine, colored yellowish green by the iron in its composition, and occurs in basalts, in mantle rocks pushed to the surface and, rarely, in meteorites. Iolite, gem cordierite, is strongly pleochroic. Kunzite is the pink to violetish purple variety of spodumene, LiAlSi₂O₆, colored by manganese and recognized around 1902. Green hiddenite and yellow triphane are the other spodumene varieties.

Turquoise is a hydrated copper aluminum phosphate, CuAl₆(PO₄)₄(OH)₈·5H₂O, with a hardness of 5–6, a refractive index of 1.610–1.650 and a specific gravity near 2.76. It forms in dry, barren country where acidic, copper-rich groundwater seeps down and reacts with minerals containing phosphorus and aluminum, leaving a porous, semitranslucent to opaque material. Deposits sit in iron-rich limonite or sandstone, whose remnants show as the dark or tan veining called matrix. The Nishapur district of Iran set the traditional standard for sky blue, and the trade calls that color Persian blue whatever the origin. CIBJO records plastic impregnation and near-colorless surface waxing as treatments commonly applied to turquoise.

Lapis lazuli is a rock, an aggregate of lazurite with calcite and pyrite and lesser diopside, amphibole, feldspar or mica. Lazurite gives the prized royal blue, and ground lapis supplied the ultramarine pigment of Renaissance painting.

Malachite, Cu₂CO₃(OH)₂, is an opaque, banded green copper carbonate found with copper ores, soft at 3.5–4.5 on the Mohs scale.

Feldspars supply three phenomenal gems. Moonstone is orthoclase intergrown with albite in stacked alternating layers; light scattered between them produces adularescence. Labradorite and sanidine can show it too, and Madagascar labradorite is sold as rainbow moonstone. Labradorite is named for Labrador in Canada, and sunstone glitters from tiny platelets, or from copper in Oregon material.

A further set of minerals appears in jewelry only occasionally and is valued mostly by collectors, usually because of softness, cleavage or an unusual optical property.

Apatite, Ca₅(PO₄)₃(F,OH,Cl), defines 5 on the Mohs scale. It comes in many colors, and neon blue-green stones from Madagascar resemble copper-bearing Paraíba tourmaline, though they scratch far more easily.

Sphene, the gem name for titanite (CaTiSiO₅), has a dispersion of 0.051, higher than diamond’s, and a birefringence of up to about 0.19, large enough that the back facets look doubled when viewed through the table. At 5–5.5 in hardness it suits pendants and earrings better than rings.

Andalusite and kyanite share the formula Al₂SiO₅ but have different crystal structures. Andalusite is strongly pleochroic, so a well-cut stone shows green and reddish brown flashes at once. Kyanite, usually blue, is unusual in having a hardness that depends on direction: about 4–4.5 along the length of a crystal and 6–7.5 across it, which makes cutting difficult.

Diopside, CaMgSi₂O₆, is best known as the intense green chrome diopside, colored by chromium and mined in Russia. Fluorite, CaF₂, defines 4 on the Mohs scale, occurs in almost every color and splits easily along four cleavage directions, so it is cut mostly for collections.

Many of the rarest gems need an unusual mix of elements, often beryllium or boron, in the right geologic setting.

Painite, a calcium zirconium aluminum borate, was discovered near Mogok in Myanmar in the early 1950s. For many years only two crystals were known, of 1.7 and 2.1 g, with a third found later in a parcel of gem rough at GIA. An 11 g crystal in 2001, finds in secondary deposits in 2004, and painite located in place at Thurein-taung and the Wetloo mine in May 2005 changed that: more than a thousand crystals and fragments have since been recovered, and most cut stones weigh 0.05–0.30 ct.

Taaffeite was noticed in October 1945 by Count Edward Charles Richard Taaffe of Dublin, who picked an unusual birefringent, spinel-like bluish violet stone out of a parcel of faceted gems. Chromium-bearing taaffeites read 1.716–1.728 for refractive index and 3.61–3.67 for density.

Grandidierite is a magnesium aluminum borosilicate, MgAl₃BSiO₉; the first transparent faceted stone, from Sri Lanka, read 1.583–1.622 with a density of 2.96. Benitoite, BaTiSi₃O₉, was found in 1907 in the New Idria district of San Benito County, has been California's state gemstone since 1985 and fluoresces strong blue under shortwave ultraviolet light. Poudretteite, known before only as tiny crystals from Mont Saint-Hilaire in Quebec, reached the market when gem rough was bought in Mogok in 2000.

A pearl forms when a mollusk coats an irritant with nacre. GIA gives pearl a composition of calcium carbonate, a hardness of 2.5–3, a refractive index of 1.52–1.69 and a specific gravity of 2.60–2.85. Light interacting with the thin nacre layers produces luster and the iridescence called orient. Natural pearls grow in the body or mantle tissue of certain mollusks, usually around a microscopic irritant, without human help of any kind. Cultured pearls grow after a technician inserts a shell bead together with a piece of mantle tissue into a host mollusk's gonad, or several pieces of mantle tissue without a bead into its mantle.

The trade recognizes four main cultured types. Akoya pearls, mostly white or cream, come from Pinctada fucata oysters farmed in Japan and China. South Sea pearls, from Pinctada maxima of Australia, Indonesia and the Philippines, are white to silver or golden, large and thick-nacred after a long growth period. Tahitian pearls, from Pinctada margaritifera around the islands of French Polynesia, may be gray, black or brown with green, blue, purple or pink overtones. Freshwater pearls, cultured mainly in China, are the most abundant and come in the widest range of sizes, shapes and colors.

Precious coral is the calcium carbonate skeleton of colonial marine animals of the family Coralliidae, in red, pink and white. Its standing under CITES is mixed. All black corals, the order Antipatharia, have been on Appendix II since 1981. Four Coralliidae species have been on Appendix III at China's request since July 2008. Mediterranean Corallium rubrum is not CITES-listed: proposals to list it were rejected in 1987, 2007 and 2010, with the advice that these long-lived species need effective local management instead.

Amber is fossilized tree resin. GIA gives it a refractive index of 1.540, a specific gravity of 1.08 and a hardness of 2–2.5, light enough to float in a saturated salt solution. Jet is a compact lignite formed from ancient wood, listed by CIBJO under the name jet or gagat.

Ammolite is the iridescent shell of fossil ammonites. CIBJO gives ammolite as the commercial name for iridescent ammonite shell and records impregnation with near-colorless hardened substances as common.

Ivory is tightly restricted. CITES placed the African elephant in Appendix I with effect from 18 January 1990, so commercial international trade is allowed only in exceptional circumstances; the populations of Botswana, Namibia and Zimbabwe (1997) and South Africa (2000) were later moved to Appendix II. In the United States, 50 CFR 17.40(e) bars selling African elephant ivory in interstate or foreign commerce, with narrow de minimis and antique exceptions.