undulatory
layers, so superimposed that a slice across a crystal shows their
triangular section distinctly, and reveals as it is turned round the
difference of colour caused by their structure and arrangement. The
alternate layers are endowed with right and left handed powers of
rotatory polarisation. The " rippled " fracture, and the feathery flaws
of amethysts are due to these fine layers.
Quartz is doubly refractive : the coloured specimens are dichroic. The indices of refraction for the yellow ray are—
and
The colours of oppositely polarized rays are, in the case of the amethyst, reddish purple and bluish purple.The
hardness of pure rock crystal is 7, and its specific gravity 2"65- A
list of specific gravities of some of the purest forms of quartz will
be useful, but it must be remembered that the dark coloured and opaque
varieties are much denser, sometimes reaching
Quartz
may be melted by the aid of the oxyhydrogen blowpipe to a limpid glass
having the specific gravity 2'2 and the hardness 5.
The
very numerous varieties of quartz cannot be classified accurately, for
many of them owe their peculiarities to intruding minerals of many
sorts. The quartz cat's-eye includes fine fibres of asbestos or
crocidolite ; avanturine quartz, minute spangles of mica ; and so on.
But the pellucid varieties group themselves near rock crystal, while
the translucent kinds may be arranged under chalcedony. The former
group includes amethyst and cairngorm, the latter sard, Chrysoprase,
and plasma. We cannot here find space for more than an alphabetical
list of the chief members of the great quartz family.
Agate --layers of chalcedony, jasper, rock crystal ; also mottled. Agate-jasper—a variety of agate containing jasper. Amethyst.—transparent, purple, honey yellow or greenish yellow. Aventuturine—transparent, with golden brown or green iridescent spangles.
