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Smoky Quartz. This
variety of quartz is often known as " smoky topaz," a misleading term,
since the mineral is not topaz. As its name implies, its color is like
that of smoked glass, all gradations occurring between a mere tinge to
color so dark as to render the mineral practically opaque. The color
often varies considerably in the same crystal, being darker and lighter
in spots. The coloring matter is undoubtedly carbonaceous and organic
in nature, for when a crystal is heated it gives off a smell of burning
organic matter, and by heating for some length of time the coloring may
be entirely burned out. At an intermediate stage in such heating the
color becomes brown or yellow, and stones of this color are often cut
as gems, and known by the name of "Spanish topaz" or "citrine." True
citrine is, however, transparent quartz with a natural yellow color.
The most remarkable crystals of smoky quartz known are some that were
found in 1868 in a hollow in granite in a locality in the Canton Uri,
Switzerland. About three thousand pounds of well-formed crystals were
there found, the largest and best of which are preserved in the Berne
Museum. The same region, and neighboring ones in the Alps, have also
furnished large quantities of smaller crystals of notable perfection in
form, and of fine quality.
The
next most important locality for smoky quartz is in the vicinity of
Pike's Peak in the State of Colorado. Here the smoky quartz occurs in
pockets in a coarse pegmatite accompanying amazon stone and other
feldspars. Kunz mentions one crystal from this locality which measured
four feet in length. Large, flawless pieces have been found, which have
been cut into facetted stones weighing a pound or more. Alexander
County, North Carolina, has also furnished much excellent smoky quartz.
Large clear crystals and masses have been found at Auburn, Maine, one
of these crystals being nearly two feet in length. These have furnished
material for balls and other objects. Smoky quartz is sometimes known
by the name of cairngorm stone, from its occurrence at Cairngorum, near
Banff, in northern Scotland. The quartz from this locality was at one
time widely distributed, and came to be regarded as the national gem of
Scotland. The cairngorm stone occurs in connection with masses of
granite, and is obtained by digging shallow pits or trenches in areas
where considerable decomposition has taken place. The stone as used in
jewelry is usually heated to give it a yellow color. Little of it is
mined at the present time.
Smoky
quartz has the physical and chemical properties of rock crystal, by
which it can be distinguished from other brown gems, such as axinite or
brown diamond. It is usually cut in the form of the brilliant or the
step cut. Being available in large, clear pieces, it is also used for
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