Gemology Glossary

Birefringence

The difference between a gemstone's highest and lowest refractive indices, observed in anisotropic (non-cubic) crystals where light is split into two rays traveling at different speeds.

Dispersion

The separation of white light into its spectral colors as it passes through a gemstone, producing the rainbow-like 'fire' visible in faceted stones.

Fluorescence

The visible light emitted by a gemstone when it is exposed to ultraviolet (UV) radiation, which ceases as soon as the UV source is removed.

Pleochroism

An optical phenomenon in which a gemstone appears to show different colors or color intensities when viewed from different crystallographic directions.

Refractive Index

A numerical value expressing how much a gemstone bends light as it passes through the stone, calculated as the ratio of the speed of light in a vacuum to the speed of light in the gem.

Transparency

The degree to which light passes through a gemstone, described on a scale from transparent (clear) to translucent (partly allowing light through) to opaque (blocking light).

Cleavage

The tendency of a crystalline mineral to break along flat, planar surfaces that correspond to directions of weakest atomic bonding in the crystal lattice.

Fracture

The way a gemstone breaks along non-planar surfaces that do not follow the crystal structure, described by the texture and shape of the broken surface.

Mohs Hardness

A relative scale from 1 to 10 that measures a mineral's resistance to scratching, where each higher number can scratch all minerals below it.

Specific Gravity

The ratio of a gemstone's weight to the weight of an equal volume of water at 4°C, providing a density measurement that is unique to each gem species.

Tenacity

A gemstone's resistance to breaking, chipping, or crushing — a measure of toughness that is distinct from hardness.

Amorphous

Lacking an ordered internal crystal structure, meaning the atoms are arranged randomly rather than in a repeating lattice pattern.

Crystal Habit

The characteristic external shape a mineral crystal tends to develop during growth, reflecting the internal atomic arrangement and conditions of formation.

Crystal System

One of the seven fundamental classifications of crystal symmetry — cubic, tetragonal, orthorhombic, hexagonal, trigonal, monoclinic, and triclinic — that describes the geometric arrangement of atoms in a mineral.

Polymorph

A mineral that shares the same chemical composition as another mineral but has a different crystal structure, resulting in distinct physical and optical properties.

Twinning

A phenomenon in which two or more crystals of the same species grow together in a symmetrical, intergrowth pattern defined by a common twin plane or axis.

Asterism

An optical phenomenon in which intersecting bands of reflected light form a star pattern — usually four- or six-rayed — across the surface of a cabochon-cut gem.

Chatoyancy

An optical phenomenon in which a luminous band of reflected light moves across the surface of a cabochon-cut gem, resembling the slit pupil of a cat's eye.

Color Change

A phenomenon in which a gemstone displays distinctly different colors under different light sources, most notably shifting between daylight (fluorescent) and incandescent light.

Luster

The quality and intensity of light reflected from the surface of a gemstone, described by its character (e.g., metallic, adamantine, vitreous, resinous, silky, pearly, waxy).

Play-of-Color

The spectacular display of spectral colors flashing across the surface of precious opal as the viewing angle changes, caused by the diffraction of light by a regular lattice of microscopic silica spheres.

Brilliant Cut

A faceting style with a circular girdle and triangular and kite-shaped facets radiating from the center, designed to maximize light return and sparkle in round stones.

Cabochon

A gemstone cut with a smooth, domed top and a flat or slightly curved bottom, with no facets, polished to a high sheen.

Calibrated Stone

A gemstone cut to precise standard dimensions (in millimeters) to fit standard commercial jewelry settings without requiring custom modification.

Faceting

The process of cutting and polishing flat, geometrically arranged faces (facets) onto a gemstone to maximize brilliance, fire, and scintillation in transparent stones.

Mixed Cut

A cutting style that combines a brilliant-cut crown with a step-cut pavilion (or vice versa), blending the sparkle of a brilliant with the color saturation of a step cut.

Step Cut

A faceting style characterized by rectangular or trapezoidal facets arranged in parallel rows (steps) on both crown and pavilion, emphasizing clarity and color over sparkle.

Inclusion

Any material enclosed within a gemstone during its formation, including crystals of other minerals, fluid-filled cavities, fractures, needles, or growth features.

Optic Character

The fundamental optical classification of a gemstone as either isotropic (singly refractive) or anisotropic (doubly refractive), and if anisotropic, whether it is uniaxial or biaxial.

Polariscope

An instrument using two polarizing filters to determine whether a gemstone is singly refractive (isotropic) or doubly refractive (anisotropic), and to detect strain or optical anomalies.

Refractometer

A gemological instrument that measures a polished gemstone's refractive index by determining the critical angle at which total internal reflection occurs between the stone and the instrument's reference glass.

Spectroscope

An optical instrument used to analyze the absorption of light by a gemstone, producing a spectrum with characteristic dark lines or bands that indicate the presence of specific elements.

Beryllium Diffusion

A treatment in which beryllium atoms are diffused into the surface layers of corundum at extremely high temperatures to alter color, most notably producing orange, yellow, or padparadscha-like sapphires.

Coating

The application of a thin surface layer of paint, lacquer, or metallic film to a gemstone's pavilion or girdle to alter or enhance its apparent color.

Fracture Filling

The injection of glass, resin, oil, or polymer into surface-reaching fractures and cavities in a gemstone to improve its apparent clarity and transparency.

Heat Treatment

The controlled heating of a gemstone to high temperatures to improve or alter its color, clarity, or both, by inducing or dissolving inclusions and changing trace element oxidation states.

Irradiation

The exposure of a gemstone to high-energy radiation (gamma rays, neutrons, or electrons) to alter its color by displacing electrons and creating color centers within the crystal lattice.

4Cs

The four universally recognized criteria used to evaluate and grade diamonds: Color, Clarity, Cut, and Carat weight.

Carat Weight

The standard unit of mass used for gemstones, equal to 0.2 grams (200 milligrams), universally adopted by the gem and jewelry trade worldwide.

Clarity Grade

A standardized assessment of the number, size, nature, position, and visibility of inclusions and blemishes in a gemstone, typically assigned on a defined scale by a gemological laboratory.

Gemological Certificate

An official document issued by an independent gemological laboratory that records the graded characteristics of a gemstone, including measurements, weight, color, clarity, cut, and any detected treatments.

Melee

Small diamonds, typically under 0.20 carats, used to accent larger center stones in jewelry settings such as pavé, channel, and halo designs.

Origin Report

A certificate issued by a gemological laboratory stating the probable geographic origin of a gemstone based on gemological testing and comparison with reference databases.