material that when cut or faceted and polished is collectibleor can be used
in jewelry.Othersare organic (such as amber, which is fossilised tree resin,
and jet,a form of coal)Some gemstones which maybe generally considered
precious or beautiful are too soft or toofragile to be used in jewelry (for eg.
single-crystal rhodochrosite) but are exhibited in museums.
Characteristics and classification
Gems are classified into different groups, species, and varieties. For example, ruby is the red variety of the species corundum, while any other color of corundum is considered sapphire. Emerald (green), aquamarine (blue), bixbite (red), goshenite (colorless), heliodor (yellow), and morganite (pink) are all varieties of the mineral species beryl.
Gems have refractive index, dispersion, specific gravity, hardness, cleavage, fracture, and lustre. They may exhibit pleochroism or double refraction. They may have luminescence and a distinctive absorption spectrum.
Material or flaws within a stone may be present as inclusions. The gem may occur in certain locations, called the "occurrence".
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