mids;
some are found with only the two pyramids, having no prism ; and others
with modifications of the angles and edges ; often in twin or double
crystals. As a rule, all crystals of quartz are more or less striated
across the planes of the prism ; sometimes very distinct, sometimes
hardly perceptible ; but never striated longitudinally down the prism,
as with the topaz or beryl. The specific gravity of all the members of
this family varies from 2-5 to 2'8 in hardness; it is
numbered '7 in the scale, and scratches glass readily. The lustre is
vitreous when transparent, inclining to resinous when opaque. The
fracture is conchoidal, and the cleavage very indistinct; the
refraction is double, and it becomes positively electric by friction.
The family of quartz may be classed under three heads :—
The vitreous—or transparent ;
The resinous—or opaque ; and
The jaspery—or the varieties having the dull colour and opacity of the jasper.
All the varieties are found of various colours and shades ; the chemical composition is pure silica, or
