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mineral,
as it occurs in large masses and often includes other minerals. The
form so used, however, has fairly definite physical and chemical
properties, and contains a very large percentage of one compound. Such
Precious or Noble Serpentine is of a rich oil green to pistachio green,
markedly translucent, with a waxy lustre ; its specific gravity is 2-5
to 2'6, and its hardness barely equal to 3. It is, however, capable of
taking a high polish. It shows no crystalline form, being always
massive (Fig. 41). It is usually a hydration product of ferro-magnesian
silicates, and it is itself a hydrated silicate of magnesium, It is found in Cornwall, in the Isle of Man, and at Portsoy, in Britain ; at Falun in Sweden ; in Saxony, Siberia, etc.
510. Sphene.
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Sphene
is only used as a gem when it occurs transparent, and of a good colour.
The colour may be green, brown, yellow, or rose red ; the lustre is
adamantine. It shows double refraction, the greatest index being 2.01
for yellow light, and the dispersion is very large. It is markedly
pleochroic, showing red, greenish-yellow, and almost colourless
images, according to the direction in which it is examined. The
specific gravity is 3.35 to 3.45, and the hardness 5 to 5-1/2. It shows
several cleavages, none very well marked. In crystalline form it is
monosymmetric, and it is usually found in embedded idiomorphic crystals
of a wedge shape, as an original constituent of granites and similar
rocks, but it may occur as a product of thermo-metamorphisru. Crystals
suitable for gem use are usually attached to the sides of cavities in
these rocks. In composition it is a titano-silicate of calcium, CaTiSi06.
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