Article by: Charles Lewton-Brain © 1986/94
This is an introductory listing of definitions and nomenclature concerning gem materials.

    A Mineral may be defined as a homogenous substance produced by the processes of inorganic nature having a chemical composition and physical properties which are constant within narrow limits. Its structure is crystalline. It is composed of the same substance throughout. Except for impurities it has the same chemical formula for all specimens of the mineral. Its atoms usually have a definite and ordered crystal structure. What makes a mineral (or an organic product) a gemstone is cultural and partly subjective: beauty, durability and rarity.     Minerals often occur in geometrical forms bounded by plane surfaces. These are crystals and the internal structure determines properties which allow the identification of the gem material; its differentiation from other minerals, imitations and sometimes synthetics.

Crystals have:

1. An orderly and symmetrical atomic structure.
2. A definite external geometrical shape bounded by plane faces.
3. Physical (and optical) properties which vary with direction.