6.
Smoky Quartz, or Cairngorm, derives its alternate name from the
Cairngorm mountains in Scotland, where it occurs in numerous places. It
is the Mormorion of Pliny, but Morion is now rather restricted to the
dark opaque varieties. In colour it varies from a pale sherry tint
through all degrees of smoky brown to almost black. Transparent to
opaque; the darker coloured specimens show a distinct dichroism, one
image being yellow-brown and the other a purer warm brown. It occurs in
crystals identical in all respects except colour to Rock Crystal.; its
commonest mode of occurrence is in fissures in granite and allied rocks
; sometimes in spaces in the outer parts of a granitic mass, probably
due to shrinkage on consolidation; in such cavities sometimes
associated with Beryl, Topaz, and crystals of Felspar.
The
colour is due to disseminated volatile compounds of hydrogen and
nitrogen, which are largely discharged at a comparatively low
temperature, so that specimens which are naturally too dark to use as
gems may, by boiling in oil, be brought to a rich colour and rendered
transparent. One crystal of very dark colour in the Royal Scottish
Museum was so treated by Professor Heddle fourteen times, but without
attaining the desired result; this, however is exceptional. Usually by
raising the temperature to 200° C. the colour is completely discharged.
Of
the Scottish localities, Cairngorm and other hills of that range
provide good crystals, also Stirling Hill in Aberdeenshire, and Beinn a
Bhuird in the same county. Very beautiful crystals are found in the
Goatfell granite in Arran. The somewhat similar granite of the Mourne
Mountains in Ireland yields beautiful specimens. The
