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—ω = 1·5442 and ε = 1·5533. 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 2·8.
Milky quartz | 2·642 | Brown cairngorm | 2·656 |
Pure rock crystal | 2·650 | Amethyst | 2·659 |
Very dark ditto | 2·662 |
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.
Avanturine—transparent, with golden brown or green iridescent spangles.