Gem science

A mineral gem has a definite chemical composition, written as a formula, and an ordered internal arrangement of atoms. Corundum is aluminum oxide (Al₂O₃); beryl is a beryllium aluminum silicate (Be₃Al₂Si₆O₁₈); diamond is carbon alone. Varieties of one species differ mainly in trace elements that substitute for the main atoms in the lattice, which is why ruby and sapphire share a single formula.

Crystallographers sort three-dimensional crystal structures into seven crystal systems: triclinic, monoclinic, orthorhombic, tetragonal, trigonal, hexagonal and cubic. Gemology describes them by the relative lengths of imaginary reference axes and the angles between them. The system governs a crystal’s outward shape (octahedra for diamond and spinel, six-sided prisms for beryl) and, more usefully for identification, its optical behavior. Cubic gems are isotropic: light travels through them at the same speed in every direction. Gems in the other six systems are anisotropic and split light into two rays. Composition alone does not fix the structure: diamond and graphite are both pure carbon, arranged differently.

Some gem materials have no long-range atomic order and so no crystal system. Glass, most gem opal and organic amber are amorphous, and like cubic crystals they are optically isotropic.

Hardness is resistance to scratching. The mineralogist Friedrich Mohs ranked ten reference minerals from talc (1) to diamond (10), each able to scratch those below it. The Mohs scale is a ranking, not a measurement with equal steps. Indentation testing makes the unevenness plain: Vickers microhardness runs about 0.14 GPa for talc, 1.5 for calcite, 12.2 for quartz and 19.6 for corundum, while diamond sits near 115. Hardness matters in jewelry because household dust contains quartz, so gems softer than Mohs 7 gradually lose their polish.

Toughness is resistance to breaking and chipping, and it is independent of hardness. Jadeite rates 6.5 to 7 and nephrite 6 to 6.5, yet both are extremely tough, which is why Neolithic knives and axe heads were often made of nephrite. Diamond, the hardest natural gem, can still split under a sharp blow: it has four perfect cleavage directions, parallel to its triangular octahedral faces. Topaz has one perfect cleavage and kunzite two. Where no cleavage exists a stone breaks by fracture; quartz and glass show curved, shell-like conchoidal fracture.

Stability is resistance to heat, light and chemicals. Opal holds up to 20% water, some kunzite fades in strong light, and pearls are etched by acids.

Specific gravity (SG) compares a gem’s weight with the weight of an equal volume of water. GIA gives diamond 3.52 (+/- 0.01). Cubic zirconia is considerably denser, so a one-carat cubic zirconia is visibly smaller than a one-carat diamond. Gemologists measure SG by weighing a stone in air and then in water, or estimate it by seeing whether the stone sinks or floats in liquids of known density.

Refractive index (RI) expresses how much light slows and bends as it enters a gem: quartz reads about 1.54, corundum 1.762 to 1.770, diamond 2.42. Stones outside the cubic system are doubly refractive, splitting light into two rays with different indices. The difference between them is the birefringence. Peridot’s birefringence of 0.035 to 0.038 is large enough that you can see two of each pavilion facet through the crown.

Dispersion is the variation of RI with wavelength, which spreads white light into spectral colors seen as fire. The chart uses the gemological B to G convention, the RI difference between two Fraunhofer lines, one in the red and one in the violet. Diamond is usually quoted at 0.044. Synthetic moissanite, which GIA measured at 0.104, far exceeds it.

Luster describes the quality of light reflected from a gem’s surface. It depends on refractive index and on how well the surface takes a polish. Diamond has adamantine luster, the brightest ordinary type; most gems, from quartz to corundum, are vitreous, or glassy. Other standard terms include resinous for amber, waxy for turquoise, pearly for pearls and some cleavage surfaces, and silky for fibrous materials.

Transparency ranges from transparent through translucent to opaque. It depends on the material itself, on inclusions that scatter light and on thickness, and in some gems it drives value: fine jadeite is prized for a glowing translucency.

Pleochroism is a change in body color with viewing direction. It occurs only in doubly refractive crystals, which absorb light differently along different optical directions. Tetragonal, hexagonal and trigonal crystals can show two colors (dichroism); orthorhombic, monoclinic and triclinic crystals can show three (trichroism). Cubic and amorphous materials show none. Tanzanite is strongly pleochroic and shows three different colors as the crystal is turned, and cutters orient the rough so that the best of them faces up through the table.

Gemologists describe color with three attributes. Hue is the basic color, such as red, blue or green, with any modifying colors. Tone is how light or dark the color appears. Saturation is its strength, from grayish or brownish to vivid.

Color arises when a gem absorbs some visible wavelengths and transmits or reflects the rest. In idiochromatic gems the coloring element is a major ingredient, as with copper in turquoise and malachite. Most gems are allochromatic: colorless when pure and colored by trace chromophores. GIA’s survey of corundum lists six of them, including Cr³⁺, Fe³⁺, V³⁺, the Fe²⁺ to Ti⁴⁺ pair and two trapped-hole pairs that give orange and yellow. Red in ruby takes several hundred to a few thousand parts per million of Cr³⁺; blue sapphire comes from intervalence charge transfer, an electron moving from Fe²⁺ to Ti⁴⁺ under light and back again.

Color centers are lattice defects that trap an electron or leave an electron gap, usually after radiation. Smoky quartz needs aluminum substituting for silicon, present in most quartz at the several hundred ppm level, before radiation can create the brown-black hole center. Amethyst is a similar center built on iron. Heating smoky quartz to about 400°C empties the traps and returns it to colorless.

Fluorescence is visible light that a gem emits while it is exposed to higher-energy radiation, usually ultraviolet (UV). Phosphorescence is a glow that continues after the radiation source is switched off. Gemologists test both with long-wave (365 nm) and short-wave (254 nm) UV lamps and record the color and strength of the response.

GIA reports that about 35% of natural colorless to faint yellow diamonds fluoresce under long-wave UV, and that blue accounts for 97% of those reactions, emitted by the N3 defect. Chromium makes ruby fluoresce red. Iron is a common quencher in colored stones, so low-iron rubies from marble deposits such as those in Myanmar fluoresce strongly.

Phosphorescence is less common and often informative. Boron-bearing Type IIb diamonds almost always phosphoresce, blue or red, under deep UV, and the Hope diamond’s red afterglow lasts several minutes. About 90% of HPHT-grown synthetic diamonds fluoresce yellow or yellow-green to short-wave UV and then give strong, long-lived phosphorescence linked to boron, which makes the test a useful screening clue. Reactions overlap between different materials, so fluorescence supports an identification rather than proving it.

Phenomenal gems show optical effects produced by their internal structure or inclusions rather than by body color alone. Most are cut en cabochon, as smooth domes without facets, which concentrates the effect.

Chatoyancy, the cat’s-eye effect, is a single bright band of light reflected from many parallel needles, fibers or hollow tubes. The band slides across the dome as the stone moves. Chrysoberyl cat’s-eye is the classic example. Asterism is the same kind of reflection from two or three sets of needles crossing one another, which produces a star. In corundum the needles are usually rutile, and GIA gives four, six or occasionally twelve rays, with six the common case.

Play-of-color in precious opal is diffraction. Silica spheres of uniform size, stacked in orderly arrays that behave as three-dimensional photonic crystals, split white light into patches of pure spectral color that change with viewing angle. Smaller spheres give blue and violet; larger ones give red, orange and green. Adularescence is the soft, floating bluish-white sheen of moonstone, which is built from alternating layers of orthoclase and albite feldspar. Light scatters between these microscopic layers and interferes, and the thinner and more uniform the layers, the stronger the effect.

Labradorescence is the broad flash of blue, green, gold or orange that sweeps across labradorite as it is tilted. On slow cooling the feldspar separates into alternating lamellae, and light reflected from successive layers interferes; the color that results follows Bragg diffraction on the stack. In one labradorite showing yellow labradorescence, the thicker lamellae averaged 129 nm and the thinner ones about 64 nm, a repeat of 193 nm that returns a reflection near 597 nm.

Aventurescence is a glittering spangle caused by small reflective platelets. In sunstone feldspar these are metallic copper or iron oxides such as hematite; aventurine quartz owes its glitter to mica platelets.

Color change is a shift in apparent hue between daylight and incandescent light. Alexandrite has an absorption band near 580 nm and transmits both green and red, so it reads green to bluish green in daylight and red to purplish red under a lamp. Some sapphires, garnets and spinels show weaker versions of the effect, and vanadium can produce it in corundum.

Iridescence is structural color made by interference of reflected or diffracted light from submicron structures. It appears in fire agate, in the surface orient of fine pearls and along thin air-filled cracks in quartz.

Inclusions are crystals, fluids, fractures and growth features enclosed in a gem. Few stones lack them, and their arrangement in each stone is effectively unique. They also record how and where the host grew, so gemologists read them to separate natural from synthetic material, detect treatment and suggest geographic origin.

Silk is a mesh of fine rutile needles in corundum. It softens color, can produce stars and alters visibly when a stone is heated, which makes it a key clue in detecting heat treatment. Needles are single elongated crystals or tubes. Feathers are fractures, named for their feathery look; surface-reaching feathers in diamond are the openings that fillers exploit. Emerald’s characteristic fissures and inclusions are called its jardin, French for garden, and dealers expect them.

Some inclusions point toward a locality. Colombian emeralds often contain three-phase inclusions: a jagged cavity holding salty liquid, a gas bubble and a tiny cube of halite (rock salt), trapped from the hot brines that deposited the emeralds. Demantoid garnets, notably from Russia’s Urals, may contain horsetails: curving sprays of fibers radiating from a minute crystal. GIA counts them among the few internal features that raise a gem’s value.

Diamond is almost pure carbon, but traces of nitrogen and boron change its color, fluorescence and electrical behavior. Scientists sort diamonds into types by these impurities, measured with Fourier-transform infrared (FTIR) spectroscopy. Nitrogen absorptions fall in the one-phonon region, roughly 1332 to 400 cm⁻¹.

Type I diamonds contain nitrogen that FTIR can measure. In Type Ia, which covers more than 95% of natural diamonds, nitrogen has migrated over geological time into adjacent pairs (A aggregates, IaA) or into groups of four atoms around a vacancy (B aggregates, IaB). In Type Ib the nitrogen sits as isolated single atoms; natural Ib stones are almost always brown, yellow or orange, and only a rare few occur. Most HPHT-grown synthetic diamonds, by contrast, are Type Ib.

Type II diamonds have no nitrogen detectable by FTIR, and they too occur very rarely in nature. Type IIa stones are often exceptionally colorless, and the largest gem diamonds, the Cullinan-like stones that formed 360 to 750 km down, routinely qualify as IIa. Type IIb diamonds contain boron, which colors them blue or gray and makes them electrically conductive. Type matters in practice: IIa is the usual type of CVD-grown diamond and the type HPHT treatment can decolorize, so IIa stones are routinely referred for laboratory testing.