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PRECIOUS STONES.
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black,
and water is given off. The specific gravity is 8"27 to 3'35, it has a
conchoidal fracture and is brittle. The hardness is only equal to 5 of
Mohs' scale ; there is a perfect rhombohedral cleavage. The streak is
green. Its crystalline form is rhombohedral and it usually occurs as
stout prisms (Fig. 29). The crystals are small. It is formed by the
decomposition of copper ores, and hence usually is associated with
other copper compounds. In composition it is a basic silicate of
copper, H20 CuO Si02. It is found in druses in a
limestone at the hill of Altyn-Tube on the Altai Mountains, also on the
Malaya River in Trans-Baikal ; also in Hungary and Chile. Good crystals
have more recently been obtained at Mindouli, east of Comba, in the
French Congo, associated with Chrysocolla. Dioptase is not extensively
used as a gem, largely on account of the difficulty of obtaining good
specimens, but also because of its liability to damage ; in colour it
is undoubtedly very fine. It is readily distinguished from Emerald by
the deeper colour, higher specific gravity and inferior hardness.
393. Idocrase.
This
mineral, which is also known as Vesuvian on account of its occurrence
on Vesuvius, is not extensively used as a gem. The colour is usually
brown, sometimes a clear green, and more rarely bright yellow or pale
blue. The gem varieties are transparent, and the lustre is vitreous.
The mineral is doubly refracting, both the ordinary and extraordinary
rays being a good deal deflected, but in a
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