Gemology term
Birefringence
The numerical difference between a doubly refractive material’s principal refractive indices.
Learn the language
Gemology glossary
Concise definitions for the scientific and trade terms used throughout The Gem Index.
Gemology term
Cleavage
A mineral’s tendency to split along specific crystallographic planes.
Gemology term
Dispersion
The separation of white light into spectral colors because refractive index varies with wavelength.
Gemology term
Fluorescence
Visible light emitted by a material while it is exposed to a suitable energy source, commonly ultraviolet radiation in gem testing.
Gemology term
Luster
The character of light reflected from a material’s surface.
Gemology term
Mohs hardness
A comparative scale of scratch resistance from 1 to 10; it does not measure toughness.
Gemology term
Pleochroism
Different body colors seen in some anisotropic gems when viewed along different crystallographic directions.
Gemology term
Refractive index
A measurement describing how light changes speed and direction as it enters a material.
Gemology term
Specific gravity
The ratio of a material’s density to the density of water under defined conditions.
Gemology term
Tenacity
A material’s response to bending, breaking, crushing, or tearing.