constituent. In composition it is an orthosilicate of magnesium and iron, 2 (MgFe)O, Si02. Olivine is easily decomposed by the mineral acids, and, according to Max Bauer, this is the rationale of the mixing of sulphuric acid with the final polishing material instead of water.
Olivine
of a pure olive-green colour, and transparent, suitable for gem
cutting, is known as Noble Chrysolite or Peridot. Although Olivine is
found in such abundance, most of it as it occurs in eruptive rocks is
in such small grains, or so cloudy or fractured, as to be unsuitable
for use ; and it. is a remarkable fact that there is hardly a locality
known where Noble Chrysolite is now to be found, even in gravel
deposits. Dr. Kunz is of the opinion that most now seen uncut, or
roughly cut, is material derived from long lost deposits, and the only
definite localities now known are in the United States, in Arizona, and
New Mexico, and there gem material is very scarce. Of the stones for
which Chrysolite may be mistaken Emerald, Prehnite, and Tourmaline are
less dense; Topaz is of the same density, but harder, and Chrysoberyl
and Corundum are denser; further, Emerald, Chrysoberyl, Corundum, and
Tourmaline are harder. Glass imitations, and the so-called Bottle
Stone, are singly refracting. Olivine is usually either step cut or
mixed cut; as a pendant stone it may be cut as a double rosette. Its
value is about equal to that of Topaz.
382. Phenakite. Phenakite
is a mineral that only occurs sparingly in Nature, and it is not very
much used as a gem. It is found colourless, wine yellow, rose pink, and
brown. The specimens used as gems are always transparent. It is doubly
