Gems are usually found in igneous rocks (mainly pegmatite dikes) and in contact metamorphic zones. The chief gems have a hardness of 8 or more on the Mohs scale, and are relatively resistant to cleavage and fracture, though some are fragile.
Gemstones are identified and characterized by their specific gravity (which also determines the size of a stone with a given weight in carats), and optical properties, especially refractive index. Gems of high refractive index show great brilliancy (also dependent on transparency and polish) and prismatic dispersion ("fire"). Other attractive optical effects include chatoyancy, dichroism (see double refraction), opalescence, and asterism – a star-shaped gleam caused by regular intrusions in the crystal lattice.
Since earliest times gems have been engraved in intaglio (where the design is cut into the stone) and cameo (where the design is in relief). Somewhat later, cutting and polishing were developed, the cabochon (rounded) cut being used. Not until the 15th century, notably in Italy, was faceting developed, now the commonest cutting style, its chief form being the brilliant cut and the step cut. Some gems are dyed, impregnated, heated, or irradiated to improve their color. Synthetic gems are made by flame-fusion or by crystallization from a melt or aqueous solution.
Related category GEOLOGY AND PLANETARY SCIENCE
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