How are Gemstones Classified?

How are Gemstones Classified?

Chemical composition

A gemstone may be a pure chemical element (diamond is essentially pure carbon), a relatively simple chemical compound (quartz is silicon dioxide, SiO2), or a more complex mixture of various compounds and elements (the garnet family includes a highly variable mix of iron, magnesium, aluminum, and calcium silicates). The great majority of familiar gem materials are oxides or silicates (i.e., they contain oxygen and perhaps silicon) and formed as crystals during the cooling of the earth's crust over past millenia.

Crystal structure

Gemstones may be formed in single or multiple discrete crystals (such as diamond), in massive collections of microscopic crystals (cryptocrystalline ) (such as chalcedony), or in amorphous (non-crystalline) masses (such as opal). In general, larger crystals were formed in areas of slow cooling of molten rock, and smaller crystals in areas of more rapid cooling. There are several classes of crystal structure based on symmetry of the resulting crystals, and there are also noncrystalline (amorphous) minerals used as gem materials. In addition, there are some organic materials (such as shell and bone) that have been used traditionally as gem materials.

Crystal systems

Cubic

Crystals in the cubic, or isometric, system have three mutually perpendicular axes of equal length. Common forms in the cubic system are the tetrahedron (4 faces), the cube (6 faces), the octahedron (8 faces), the dodecadehedron (12 faces), the trapezohedron (24 faces), and the hexoctahedron (48 faces). Gemstones occurring in cubic crystal forms include diamond, the garnets, pyrite, and spinel.

Hexagonal

Crystals in the hexagonal system have four axes, three of which are of equal length and intersect at 60 degree angles within a plane, and the fourth of which is perpendicular to the plane of the other three. Gemstones occurring in hexagonal crystals include beryl, corundum, quartz, and tourmaline. Some crystallographers further identify two subdivisions of hexagonal crystals: trigonal (corundum) and rhombohedral (quartz).

Tetragonal

Tetragonal crystals have three axes intersecting at 90 degree angles, two of which are of equal length. Examples include zircon, rutile, and scapolite.

Orthorhombic

Orthorhombic crystals have three axes at 90 degree angles , all of which have different lengths. A typical example is topaz.

Monoclinic

Monoclinic crystals have three axes of unequal length, two of which intersect at an angle other than 90 degrees, and both perpendicular to the third. Jadeite and nephrite are common examples.

Triclinic

Triclinic crystals have three axes, all of unequal length and intersecting at angles other than 90 degrees. Examples include labradorite and microcline feldspar.

Optical characteristics

Optical characteristics of gemstones are primarily derived from their chemical composition and crystal structure.

Color

Color is the apparent result of selective absorption or transmission of different frequencies of visible light. Color can be described as the combination of three characteristics: hue, tone, and intensity. Hue is a function of the frequency of light and is described by familiar terms such as red, orange, yellow, blue, green, indigo, and violet. Tone is a variation from very light to very dark. Intensity is a measure of saturation, or purity, of a color. The typical human eye can identify approximately 150 pure hues, but around one million colors. The differences among colors may be immediately obvious or so subtle that direct comparison under controlled conditions is required to discern them. Color acuity is also highly affected by fatigue, diet, and other factors, so it is unwise to attempt judging subtle color differences in gemstones such as diamond without attention to the physical and emotional condition of the observer, as well as properly graded comparison stones and careful control of lighting conditions.

Pleochroism is the apparent change in color of a doubly refractive gemstone when viewed through different directions of the crystal structure. In most cases, the color variations are not obvious to the unaided eye and must be viewed through a polariscope or dichroscope, but in some cases, the pleochroic colors are strikingly obvious. For example, many green tourmalines appear black through the C axis of the crystal, and iolite shows a striking combination of blue-violet and near colorless. Dichroism refers to the display of two ("di") pleochroic colors in a gemstone.

Alexandrite-like color change, or photochroism, is the marked change in perceived color of a gemstone under different lighting conditions. As the name implies, the most famous example appears in alexandrite, a form of chrysoberyl that typically appears blue or green in daylight and red or purplish in incandescent light, but similar color changes may be observed in sapphire, garnet, and tourmaline. The phenomenon is due to selective absortion of different wavelengths of light, and the predominance or absence of those wavelengths in the prevailing light (incandescent light has proportionately higher quantities of reddish wavelengths and less of blue or green).

Optic Character

Gemstones may affect the passage of light differently through different directions in the crystal structure. If the velocity of light is constant through all directions in the stone, the stone is said to be singly refractive, or isochroic, and has one refractive index. This is characteristic of isometric crystals. If the velocity of light varies with direction, the stone is doubly refractive, or anisotropic, and has two refractive indices. In anisotropic materials, light is separated into two polarized components, the ordinary ray and the extraordinary ray. Anisotropic materials can be further characterized as uniaxial, biaxial positive, and biaxial negative.

Amorphous (non-crystalline) materials, such as opal, amber, and glass, may scatter light in unusual directions due to internal stress and display a phenomenon known as anomalous double refraction.

Refractive Index

Refractive index, or R.I., is the ratio of the velocity of light in air to the velocity of light through a transparent material. If light passes from air into a transparent material at an angle of incidence other than a 90 degree angle, it is deflected at a different angle (the coincident angle) according to the R.I. Gemstones with higher R.I. are generally more brilliant than those with low R.I. For example, diamond has an R.I. of about 2.4; quartz, about 1.54-1.55. The R.I. of most gemstones is easily measured using a simple optical instrument known as a refractometer.

Birefringence

Birefringence is the difference in value between the highest and lowest refractive indices in a doubly refractive (anisotropic) material. Depending on the orientation of a faceted stone, this can result in a "fuzzy" appearance and apparent doubling of facets viewed through the stone.

Dispersion

Dispersion is the ability of a gemstone to separate light into its component colors; that is, the quality of passing different wavelengths of light at different velocities. Dispersion is the quality in a diamond that produces sparkles of color in an otherwise colorless stone. Quartz, which has a dispersion of 0.013, shows much less of this effect than diamond, which has a dispersion of 0.044. Diamond, in turn, shows much less color play than sphalerite, which has a dispersion of 0.156.

Fluorescence

Many materials are fluorescent. That is, when exposed to ultraviolet light or X-rays, they transform some of the incoming energy into visible light. The color and intensity of the fluorescence is often indicative, but not conclusive, of the identity of the material. For example, natural yellow sapphires from Ceylon show a distinctive apricot-colored fluorescence, while synthetic yellow sapphires generally show no fluorescence or a dull red when exposed to long-wave ultraviolet (UV) light. Most natural emeralds are inert (non-fluorescent) under long-wave UV, and most synthetic emeralds show a moderate to strong red fluorescence. Because of the prominent exceptions, this test alone is inconclusive.

Phosphorescence

If a fluorescent material continues to emit light after the exciting UV or X-ray light is removed, it is said to be phosphorescent. This phenomenon usually lasts only a few seconds but may occasionally persist for much longer periods. This is a relatively rare characteristic in gemstones.

Clarity

Gemstones can vary from complete opacity to lucid clarity and may contain few or many inclusions such as crystals of other minerals, gas- or liquid-filled cavities, or even insects! (Large, perfectly preserved insect specimens in amber are highly prized.) In some gemstones, such as emerald, certain inclusions are highly distinctive and can be used as reliable indicators of identity. A gemological microscope (a binocular microscope with a typical magnification of 10X to 40X) is one of the most useful tools in identifying many gemstones, as well as grading them on relative clarity.

Specific gravity

Gem materials vary greatly in density -- amber may float in salt water (density near that of water), while hematite is more than five times the density of water. This is why two different gemstones may have the same size but different weights and vice versa -- a one carat round brilliant diamond of typical proportions will be approximately 6.5 mm in diameter, while a round brilliant ruby of the same size (6.5 mm in diameter) and proportions will weigh approximately 1.55 carats. Generally, gemologists refer to specific gravity, or relative density -- the ratio of the density of a gemstone relative to that of water.

Durability

The two most familiar qualities of durability -- hardness and toughness -- are often misunderstood. Hardness is resistance to scratching or piercing. Toughness is resistance to breakage. The combination of the two largely defines the durability of a gemstone. Diamond is the hardest naturally occurring material and is also quite tough; however, it can be broken by a hard blow. Jadeite and nephrite (the jades) are much softer and relatively easy to scratch but are perhaps the toughest gem materials. Hardness is often represented on the Mohs scale, a nonlinear scale of scratch resistance varying from 1 (talc) to 10 (diamond). The Mohs scale can be misleading -- there is a much greater difference in hardness between 9 (corundum) and 10 (diamond) than between 9 and 1 (talc). More precise, and less familiar, measurements of hardness are done using other systems, such as the Knoop scale of resistance to indentation. Because of the likelihood of physical damage, hardness tests are NOT recommended for gem identification. Resistance to chemical degradation or to changes in temperature or humidity are important. Turquoise is often quite porous and can be discolored by exposure to oils. Opals are heat-sensitive and have a high water content; sudden temperature changes or extremely dry conditions can cause them to crack or craze.

Thermal conductivity (the ability to conduct heat) is very low in most gemstones but is extremely high in diamond (from 1.6 to 4.8 times as great as in pure silver!). This unusual property of diamond is the basis for several popular diagnostic probes that are used to distinguish diamond from its numerous imitations.