This glossary defines the words gemologists use to describe, identify and grade gemstones. Each entry is short, and terms with a full page on the site link to it. For metals and jewelry design terms, see the jewelry glossary and the metallurgy glossary.
A
Adamantine luster
The very bright, almost metallic surface shine of diamond and a few other high refractive index gems. It is the highest luster grade short of metallic.
Adularescence
A soft, floating glow that moves across a gem as it turns, caused by light scattering between thin layers of feldspar. It is the effect seen in moonstone.
Allochromatic
Describes a gem whose color comes from trace impurities rather than its defining chemistry. Corundum is colorless when pure and becomes ruby or sapphire through traces of chromium, iron or titanium; see color in gemstones.
Alluvial deposit
Gem-bearing gravel laid down by rivers after the gems weathered out of their host rock. Many sapphires, rubies and diamonds are recovered this way.
Amorphous
Lacking an ordered crystal structure. Opal, glass, amber and obsidian are amorphous.
Anisotropic
Describes a material whose optical properties vary with direction, so it splits light into two rays. All crystals outside the cubic system are anisotropic.
Asterism
A star of light bands across a cabochon, caused by aligned needle-like inclusions. Star sapphire and star ruby are the best-known examples; see gemstone optics.
Aventurescence
Glittering reflections from small platelet inclusions, such as fuchsite in green aventurine or hematite in sunstone.
B
Biaxial
Describes doubly refractive crystals with two optic axes, found in the orthorhombic, monoclinic and triclinic systems. Peridot, topaz and tanzanite are biaxial.
Birefringence
The difference between a doubly refractive gem's highest and lowest refractive index. High birefringence produces visible facet doubling, as in zircon; see birefringence and double refraction.
Brilliance
The white light returned to the eye from inside a faceted gem. It depends on refractive index and on cut proportions.
C
Cabochon
A gem cut with a smooth, domed top and no facets. Cabochons suit opaque, translucent and phenomenal gems.
Carat
The unit of weight for gemstones, equal to 0.2 grams and divided into 100 points. See diamond carat weight.
Chatoyancy
A single bright line, the cat's eye effect, that crosses a cabochon cut from material with parallel fibers or needles. Cat's eye chrysoberyl and tiger's eye show it.
Chelsea filter
A hand filter that passes only deep red and yellow-green light. Some gems colored by chromium, such as many emeralds, appear red through it.
Cleavage
The tendency of some crystals to split cleanly along flat planes of weak atomic bonding. Topaz, diamond and kunzite all have cleavage; see cleavage in gemstones.
Color center
A defect in a crystal lattice, often created by natural or artificial radiation, that absorbs light and causes color. Smoky quartz and many blue topazes owe their color to color centers.
Color change
A shift in a gem's color between daylight and incandescent light. Alexandrite is the classic example, moving from green to red.
Conchoidal fracture
A curved, shell-like break surface typical of quartz, glass and many other gems. See fracture in gemstones.
Crown
The part of a faceted gem above the girdle, including the table. The crown facets disperse light into fire.
Crystal system
One of the seven groups (cubic, tetragonal, hexagonal, trigonal, orthorhombic, monoclinic and triclinic) into which crystals are sorted by symmetry. See crystal systems in gemstones.
Culet
The point or small facet at the bottom of a gem's pavilion. Old cuts often have a large, open culet.
D
Diaphaneity
How much light passes through a gem, from transparent to opaque. See diaphaneity.
Dichroscope
A small instrument that shows a gem's pleochroic colors side by side. It helps separate doubly refractive gems from singly refractive ones.
Diffusion treatment
Heating a gem with chemicals so color-causing elements move into it from the surface. Beryllium diffusion of sapphire is one form; see sapphire treatments.
Dispersion
The splitting of white light into spectral colors as it passes through a gem. Diamond's dispersion is 0.044, and demantoid and titanite are higher.
Doublet
An assembled stone made of two parts glued together, such as a thin slice of opal on a dark backing. Doublets must be disclosed.
F
Fancy color
A diamond with color beyond the normal D to Z range, or with a hue other than yellow or brown. See fancy colored diamonds.
Feather
A gemologist's term for a fracture inside a gem, named for its feathery look under magnification.
Fire
The flashes of spectral color produced by dispersion. Fire is most visible in gems with high dispersion and a well-proportioned crown.
Fluorescence
Visible light emitted by a gem while it is exposed to ultraviolet light. Many diamonds fluoresce blue, and some rubies glow red.
Fracture filling
A treatment that fills surface-reaching fractures with glass, resin or oil to make them less visible. It is common in emerald and found in some diamonds and rubies.
G
Gemology
The science of identifying, grading and studying gemstones. See what is gemology.
Girdle
The narrow band around the widest part of a faceted gem, where crown meets pavilion. Settings usually grip the girdle.
H
Habit
The typical outward shape a mineral crystal grows in, such as the octahedron of diamond or the hexagonal prism of beryl. See crystal habit.
Hardness
A gem's resistance to scratching, usually stated on the Mohs hardness scale.
Heat treatment
Controlled heating used to improve a gem's color or clarity. It is routine for sapphire, ruby, tanzanite and much citrine.
I
Idiochromatic
Describes a gem whose color comes from an element that is part of its chemical formula. Peridot (iron) and malachite (copper) are idiochromatic.
Imitation
A material made to look like a gem but with different chemistry and properties. Glass imitating emerald is one example.
Inclusion
Any feature inside a gem, such as a crystal, fluid, needle or fracture. Inclusions can help prove whether a stone is natural and where it came from.
Iridescence
Rainbow colors caused by light interference in thin layers or fine structures. It appears in fire agate and in some coatings.
Isotropic
Describes a material with the same optical properties in every direction, giving a single refractive index. Cubic minerals such as diamond, spinel and garnet, and amorphous materials such as glass, are isotropic.
J
Jardin
French for "garden," the trade term for the mossy inclusions typical of emerald.
K
Kimberlite
A volcanic rock that carries diamonds from deep in the mantle to the surface in carrot-shaped pipes. See diamond geology.
L
Labradorescence
Flashes of blue, green, gold or other colors from inside labradorite, caused by light interference in thin layers. See labradorite.
Loupe
A hand magnifier, normally 10x, used to examine gems. A 10x loupe is the standard for diamond clarity grading.
Luster
The way light reflects from a gem's surface. Common terms include adamantine, vitreous, resinous, pearly and silky.
M
Mohs scale
A ten-step scale of scratch hardness devised by Friedrich Mohs in 1812, from talc at 1 to diamond at 10. It ranks order only and does not measure equal steps.
N
Nacre
The layered material of aragonite platelets and organic conchiolin that forms the surface of pearls and mother-of-pearl. See what pearls are made of.
O
Optic character
Whether a gem is singly refractive, uniaxial or biaxial. It is a standard identification test made with a polariscope or refractometer.
Orient
The soft iridescent shimmer seen on or just below the surface of some fine pearls.
P
Padparadscha
A rare pinkish orange to orangy pink sapphire, named from the Sinhalese word for lotus blossom.
Pavilion
The lower part of a faceted gem, below the girdle. Pavilion angles control how much light returns through the top.
Pegmatite
A very coarse-grained igneous rock that forms large crystals of tourmaline, beryl, topaz and spodumene. See igneous rocks and gemstones.
Phenomenal gem
A gem that shows an optical effect such as asterism, chatoyancy, play-of-color or adularescence.
Play-of-color
Patches of spectral color in precious opal that change as the stone moves, caused by diffraction from ordered silica spheres. See opal.
Pleochroism
Different colors seen along different directions in a doubly refractive gem. Two colors is dichroism and three is trichroism; see pleochroism.
Polariscope
An instrument with two polarizing filters used to tell singly refractive from doubly refractive gems and to detect strain.
R
Refractive index
A measure of how much a gem slows and bends light, read on a refractometer. Diamond's RI is about 2.42; see refractive index of gemstones.
Refractometer
An instrument that measures refractive index by placing a polished facet on a glass hemisphere with a contact liquid. Standard units read up to about 1.81.
Rough
Uncut gem material as it comes from the ground.
S
Schiller
A sheen or flash of color from inclusions or layers inside a gem, such as the copper schiller in Oregon sunstone.
Silk
Fine, needle-like inclusions, usually rutile, that give a soft sheen to some corundum and garnet. Dense silk can produce asterism.
Simulant
Any material used to imitate another gem, natural or man-made. Cubic zirconia and moissanite are diamond simulants.
Specific gravity
The ratio of a gem's density to the density of water. It helps separate look-alikes; see specific gravity of gemstones.
Spectroscope
An instrument that splits light into a spectrum so a gemologist can see the dark absorption lines a gem produces.
Synthetic
A man-made material with the same chemistry, crystal structure and properties as its natural counterpart. Lab-grown diamond and synthetic ruby are examples; see lab-grown diamonds.
T
Table
The large flat facet on top of a faceted gem.
Toughness
A gem's resistance to breaking, chipping and cracking, which is separate from hardness. Jade is very tough though only moderately hard; see gemstone durability.
Treatment
Any process other than cutting and polishing that changes a gem's look or durability. See gemstone treatments explained.
Trichroism
Pleochroism showing three different colors along different directions, as in tanzanite and iolite.
Triplet
An assembled stone of three layers, such as a thin opal slice with a dark backing and a clear quartz or glass cap.
Twinning
Intergrowth of two or more crystals of the same mineral in a set orientation. Twinning planes can show as lines inside a gem.
Type IIa
A diamond category with almost no nitrogen. Many of the large historic Golconda diamonds are Type IIa; see diamond chemistry and formation.
U
Uniaxial
Describes doubly refractive crystals with one optic axis, found in the tetragonal, hexagonal and trigonal systems. Quartz, corundum and tourmaline are uniaxial.
V
Vitreous luster
The glassy luster shown by most gems, including quartz, beryl and corundum.
W
Window
A washed-out area in the center of a faceted gem where light passes straight through instead of reflecting back. It usually comes from a shallow pavilion.
Z
Zoning
Uneven distribution of color in bands or patches within a gem. Color zoning is common in sapphire and amethyst.