Glossary

Gemmological terminology definitions.

A

Adularescence : A billowy, floating light effect seen in moonstone, caused by light scattering from thin layers of orthoclase and albite feldspar.

Allochromatic : A mineral that is colourless when pure, with colour caused by trace impurities (e.g., corundum, beryl).

Anomalous Double Refraction (ADR) : Birefringence observed in normally isotropic materials due to strain, common in garnet and spinel.

Asterism : A star effect caused by light reflecting from oriented needle-like inclusions, typically silk in corundum.

B

Birefringence : The numerical difference between the highest and lowest refractive index values in a doubly refractive gem.

Brilliance : The amount of light returned from a gemstone through its crown.

C

Cabochon : A gem cut with a domed top and flat or domed back, typically used for opaque gems or those displaying phenomena.

Carat : Unit of weight for gemstones, equal to 0.2 grams or 200 milligrams.

Chatoyancy : The cat's eye effect, a single bright band of light across a domed cabochon.

Chromophore : An element responsible for colour in gemstones (e.g., chromium in ruby, iron in aquamarine).

Cleavage : The tendency of a mineral to break along specific crystallographic planes of atomic weakness.

Crown : The upper portion of a faceted gemstone, above the girdle.

D

Dichroism : The property of showing two different colours when viewed along different crystallographic axes.

Dispersion : The separation of white light into spectral colours as it passes through a material; causes "fire" in diamonds.

Doublet : A composite stone made of two parts cemented or fused together.

E

Extinction : In a polariscope, the darkness observed when a birefringent gem is rotated to certain positions.

F

Facet : A flat, polished surface on a cut gemstone.

Fire : Flashes of spectral colours seen in a gemstone due to dispersion.

Fluorescence : The emission of visible light by a material when exposed to ultraviolet radiation.

Fracture : The way a mineral breaks along surfaces other than cleavage planes.

G

Girdle : The narrow rim around a cut gemstone that separates the crown from the pavilion.

Growth zoning : Colour or inclusion patterns that reflect a gem's growth history.

H

Habit : The characteristic external shape of a crystal.

Hardness : Resistance to scratching, measured on the Mohs scale.

I

Idiochromatic : A mineral whose colour is caused by essential elements in its chemical composition (e.g., peridot, turquoise).

Inclusion : Any material trapped within a gemstone during or after formation.

Isotropic : Having the same optical properties in all directions; singly refractive.

J

Jardín : French for "garden"; refers to the characteristic inclusion pattern in emerald.

L

Labradorescence : The play of spectral colours seen in labradorite, caused by light interference from lamellar twinning.

Lustre : The quality and quantity of light reflected from a gem's surface.

M

Miller indices : A notation system for identifying crystal faces using three or four integers in curly braces.

Mohs scale : The 10-point relative hardness scale used for minerals.

O

Optical character : Whether a gem is singly refractive (isotropic) or doubly refractive (anisotropic), and if doubly refractive, whether uniaxial or biaxial.

Optic sign : Whether a uniaxial or biaxial gem is optically positive (+) or negative (-).

P

Pavilion : The lower portion of a faceted gemstone, below the girdle.

Phenomenon : Special optical effects in gemstones such as asterism, chatoyancy, or play of colour.

Play of colour : The spectral colour flashes seen in precious opal, caused by diffraction from silica spheres.

Pleochroism : The property of showing different colours when viewed along different crystallographic axes.

Point group : One of the 32 classes of crystal symmetry.

R

Refractive index (RI) : A measure of how much light slows and bends when entering a material.

S

Silk : Fine, needle-like rutile inclusions in corundum that can cause asterism.

Simulant : Any material used to imitate the appearance of a gemstone.

Specific gravity (SG) : The ratio of a material's density to that of water.

Synthetic : A man-made material with essentially the same chemical composition and crystal structure as its natural counterpart.

T

Table : The large, flat facet on top of a cut gemstone.

Toughness : Resistance to breaking, chipping, or cracking.

Trichroism : The property of showing three different colours when viewed along different crystallographic axes.

Triplet : A composite stone made of three parts cemented or fused together.

Twinning : The intergrowth of two or more crystals of the same mineral in a specific crystallographic relationship.

U

Uniaxial : Having one optic axis; characteristic of tetragonal, hexagonal, and trigonal crystals.

V

Vitreous : Glass-like lustre.

W

Window : An area in a cut gemstone where light passes straight through without being reflected back, appearing pale or washed out.

Z

Zoning : Variation in colour intensity within a gemstone, typically following growth patterns.