Gemstone Glossary

This page describes common definitions for words you will find as you study gemology.

Adularescence
Adularescence is an optical phenomenon found in moonstones, also called schiller. It is usually a milky or silvery white (and more rarely orange or blue) with a "billowy" quality.
Amphibole
Amphibole is the name of an important group of generally dark-colored, inosilicate minerals, forming prism or needlelike crystals. Amphiboles can be green, black, colorless, white, yellow, blue, or brown, such as tremolite, actinolite, tourmaline, hornblende, and epidote.
Asterism
A star-like phenomenon caused by light reflecting from tiny fiber-like inclusions perpendicular to the crystal faces.
Aventurescence
An optical effect characteristic of some translucent minerals, such as aventurine, caused by the presence of tiny, aligned inclusions of other minerals and manifesting itself as brightly colored reflections.
Chatoyancy
Chatoyancy is the "cat's eye" effect caused by light scattering off thin parallel fibrous or needle-like inclusions.
Chromophore
A chromophore is the part of a molecule responsible for its color. The color arises when a molecule absorbs certain wavelengths of visible light and transmits or reflects others.
Cryptocrystalline
Aggregates of randomly orientated small crystals or crystalline fibers which cannot be seen with the eye or standard optical microscope. Also called microcrystalline.
Dendritic
Crystals that grow having a branching or tree-like growth pattern.
Exsolution
Exsolution is the process through which an initially homogeneous solid solution separates into at least two different crystalline minerals without the addition or removal of any materials. In most cases, it occurs upon cooling below the temperature of mutual solubility or stability of the solution. More information on exsolution.
Labradorescence
A play of colors or colored reflections exhibited especially by labradorite and caused by internal structures that selectively reflect only certain colors.
Lamellar Twinning
In crystallography, lamellae are a special kind of crystal twinning where many twin individuals each have a tabular or plate-like appearance. Lamellar twinning is common within the plagioclase feldspars.
Massive
Crystals without external crystal shape or masses of smaller crystals.
Matrix
Groundmass. The solid matter in which a fossil or crystal is imbedded.
Polycrystalline
Aggregates of randomly orientated small crystals or crystalline fibers which can be seen under magnification and sometimes with the eye.
Phenocryst
A large or conspicuous crystal in a porphyritic rock, distinct from the groundmass.
Polymorph
Polymorphism is the ability of a solid material to exist in more than one form or crystal structure.
Porphyritic
Relating to or denoting a rock texture, typically found in volcanic rocks, containing distinct crystals or crystalline particles embedded in a fine-grained groundmass.
Schiller
Schiller (from the German term for the shimmer of color) is an unusual iridescent or metallic lustre or billowy light in some minerals caused by internal reflection from inclusions such as gas cavities or mineral intergrowths.
Tenebrescence
Tenebrescence, also known as reversible photochromism, is the ability of minerals to change color in response to the application or absence of certain wavelengths of light, such as white light or UV radiation.

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