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idiochromatic
self-coloured, fromm greek idio = peculiar, one´s own and chroma = colour
gemstones in which the colouring elements are constituent parts of the chemical formula. examples: malachite (CuCO₃) and dioptase (H₂CuSiO₄) are coloured green by copper, rhodonite (MnSiO₃) gets its rose-red colour from manganese.
opposite of allochromatic

imitations
natural or arftificial substances that imitate natural gemstones e.g. glas or plastics. contrary to synthetic gems imitations do not have the same physical and optical properties as their natural counterparts.

indigolite
also: indicolite, tradename for blue tourmaline, usually, but not always, elbaite

isotropic
singly refractive, opposite: anisotropic

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