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PRECIOUS STONES.
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mineral
be examined under the polariscope with the Nicol's prisms crossed, on
rotating the specimen the field becomes lighter, unless viewed along
the optic axis, when all remains dark.
It
is moderately diathermanous, and conducts heat well; but better along
the optic axis than at right angles to it. It is very infusible,
requiring the temperature of the oxy-hydrogen blowpipe to fuse it. A
difference of electric potential is developed by heat, and also by
pressure. Its electrical resistance is bigh, and hence it is useful as
an insulator. Fine spun threads of Quartz retain their insulating
properties well, even in moist surroundings.
The specific gravity in pure forms is about 2"65 ; in some crypto-crystalline forms 2-60.
Fracture:
Conchoidal in a marked degree in well crystallised specimens; some
massive varieties show a splintery or flat fracture. Rarely, a cleavage
parallel to the faces of the positive and negative rhombohedra is seen;
the frangibility varies from brittle to tough. The hardness is the
standard 7 of Mohs' scale. The streak left on a harder substance is
white in the colourless varieties, or of a pale colour in other kinds.
Crystalline
Form : Quartz belongs to the rhombohedral division of the hexagonal
system, and thus, though crystals appear purely hexagonal, it is found
that alternate faces have different physical properties ; often the
rhombohedral character is marked. The commonest type of crystal is a
prism terminated at one or both ends by two rhombohedra which often
look like a hexagonal pyramid. The prism faces may be long or entirely
absent; when present they often show transverse marking. It is
noteworthy that the
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