Gemology glossary

An optical phenomenon in which two or more bright bands intersect to form a star-like pattern, commonly from oriented internal features in a cabochon.

The numerical difference between principal refractive indices in an anisotropic material.

A bright, mobile band of reflected or scattered light, often produced by aligned internal features and displayed by a cabochon.

An element or structural feature responsible for selective light absorption and resulting color in a material.

A mineral’s tendency to split along particular crystallographic planes where bonding is comparatively weak.

A lattice defect that absorbs selected wavelengths of light and can produce color in a crystal.

One of the major symmetry-based classifications used to organize crystalline materials.

The variation of refractive index with wavelength, which can separate white light into spectral colors.

Visible light emitted by a material while it is exposed to an energy source, commonly ultraviolet radiation in gem testing.

The way a material breaks where the break does not follow a cleavage plane.

A natural or manufactured material valued for beauty, rarity, durability, or ornamental use and suitable for personal adornment or collecting.

The evidence-led process of determining a gem material by combining observations and tests.

A foreign material or cavity enclosed within a host gem material.

The character of light reflected from a material’s surface.

A naturally occurring solid with a characteristic chemical composition and an ordered atomic structure.

A mineral category defined by a distinct chemical composition and crystal structure.

A named variation within a mineral species, commonly distinguished by color, optical effect, or another characteristic.

A relative scale of resistance to scratching, ranked from 1 to 10 using reference minerals.

A classification describing how a crystalline material interacts with polarized light according to its symmetry and principal refractive indices.

Different body colors observed in some anisotropic materials when viewed along different crystallographic directions.

Refractive index (RI) is a ratio describing how quickly light propagates in a material compared with a reference medium. At a boundary, that change in propagation speed is associated with refraction. In gemology, a refractometer reading provides a useful diagnostic property that is evaluated with optical character, birefringence, specific gravity, spectroscopy, inclusions, and other observations—not as proof of identity by itself.

A gemological instrument used to determine refractive index from critical-angle behavior at an optical contact.

A natural or manufactured material used to imitate the appearance of another gem while differing in composition, structure, or properties.

The ratio of a material’s density to the density of water under specified conditions.

A laboratory-grown material with essentially the same chemical, crystal-structural, optical, and physical characteristics as its natural counterpart.

A material’s characteristic response to mechanical stress, including brittle, malleable, sectile, or elastic behavior.

A material’s resistance to breaking, chipping, or fracturing when force is applied.

The degree to which a material transmits light, commonly described as transparent, translucent, or opaque.