Rock Crystal.
The
first variety of vitreous quartz is the colourless one called Rock
Crystal, which is found in beautifully-formed crystals—either detached
or in groups, and in rounded water-worn pebbles—in various localities
in almost every part of the globe :—in the Isle of Wight, at Bristol,
on Snowdon, in Derbyshire, Cornwall, Cumberland, and on Cairngorm, in
Scotland ; in the mountains in Wicklow and Donegal, Ireland; in Savoy
and Dauphine; in the Carrara Mountains, in Hungary, and on the Alps,
etc. It is also met with in the East Indies, Ceylon, Brazil, Quito,
Canada, and Australia.
Crystal
sometimes contains admixture of mica, rutile, tourmaline, topaz,
asbestos, bitumen, and other foreign matters, and is often found with a
greenish mineral called chlorite; occasionally possessing a cavity
containing water, with an air-bubble in it, which moves as the crystal
is turned about; sometimes containing gases and liquids. Crystals of a
very large size are occasionally met with, but they are rarely perfect.
One specimen, in the Jardin des Plantes at Paris, measures three feet
in diameter, and weighs eight hundred pounds. Geodes, or hollow
globular masses of quartz, are found in many trap rocks; some of them
occur of as large a size as two feet in diameter : small specimens are
found at Clifton, near Bristol, where they are known as potato stones.
This stone, under the name of pebble, is used by
