turning
the specimen about, however, a wave of milky white light is seen to
cross the stone, due to reflection from the innumerable fibres of
Asbestos. In some cases the Asbestos has been removed from the more
durable Quartz by weathering, and thus a series of minute tubes are
left. These, however, give the same optical effect.
Most specimens are from India and Ceylon ; in the latter locality the mineral occurs in the gem gravels as pebbles.
It is usually cut en cabochon, with
the lower surface parallel to the fibres ; on turning the stone the
band of light is seen to move across; the ideal is to have the light
band as narrow and bright as possible.
11.
Aventurine is a sub-transparent to sub-translucent Quartz, containing
numerous scales of some glittering mineral, which may be Mica,
Haematite, Limonite, or other mineral. The Quartz is colourless, but
the enclosures are usually a golden or muddy brown, sometimes silvery
or green, rarely blue. The flakes of the enclosed mineral are often
arranged in a definite way, so that on turning the specimen a brilliant
metallic reflection is obtained. It is sometimes distinguished from the
Aventurine varieties of the Felspars by calling it Quartz Aventurine.
When cut as a gem it is usually given a low, rounded surface ; more
often it is used for the production of vases and similar objects.
Amongst
the localities where it is found Pliny mentions it, under the term
Corallachates, as coming from Crete. There is a possibility that the
Sandaster of Pliny may have been Aventurine in part. In Scotland it
occurs on Ben Hope, in Sutherland, coloured by a red Mica and red
Zircon ; also on Ben Eibhinn, in Inverness-shire. In Spain it is found
at Cape de Gatte; at Nantes, in France; in
