No serious discussion of crystallography should begin without a cursory examination of the science of Mineralogy. One of the characteristics of a mineral is ..
" A naturally occurring, inorganic material of constant chemical composition and a definite crystalline structure."
Exceptions are amorphous minerals. "Lacking a systematic arrangement of atoms"
Examples of amorphos minerals are:. Alabaster, Moldavite, Obsidian, Odontolite, Onyx Marble, Steatite, Travertine.
Another classification of gemstones are those which are rocks. Technically, a rock is, "A mechanical integration of two or more minerals" - Liddicoat. Examples are Lapis and Pyrite
Not all gemstones are minerals. Some are organics, Examples of organic gemstones are Amber, Coral, Ivory, Jet, Pearl and Shell
Mineralogists describe minerals in much the same way a biologist describes animals and plants.
- Group - Two or more gem minerals similar in structure but with slightly different chemical properties. (ex. one chemical element replaces another in the chemical formula)
- Species - Individual members of a group. All varieties have the same crystal structure and chemical composition (except trace elements that determine color)
- Variety - A sub-division of a species
Crystalline materials can be classified in one of the six crystal systems
- Isometric/Cubic - Diamond & Spinel
- Hexagonal - Beryl & Corundum
- Trigonal - Zircon
- Orthorhombic - Topaz & Chrysoberyl
- Monoclinic - Jades & precious Moonstone
- Triclinic - Turquoise & Labradorite feldspar
Crystal systems tend to be either singly refractive or multi-refractive.
- Single Refractive - Crystals in the systems have only one optic axis, so light is not divided into 2 rays when it enters the crystal
- Multi Refractive - Crystals in the systems have two
2 or more optic axes, so light is divided into 2 or more rays when it enters
Each ray of light is plane polarized. Light passing through the crystal vibrates on two planes perpendicular to each other. In one plane, the velocity of light is reduced more than the other.
Axes are commonly referred to as
- "C" axis = direction of single refraction in a doubly refractive crystal
- "A" axis = direction of double refraction in a doubly refractive crystal
Generally speaking, the best color will be seen down the "C" axis. So most gems will be cut with the table perpendicular to the "C" axis. Only one color will be observed.
Gems cut off perpendicular will normally show a mix of primary and secondary colors. That color mix will be less desirable. Observing the A axis with a calcite dichroscope will reveal the primary and secondary rays separated