of
Geierstein," in which the baleful influence of the opal plays a
prominent part; and it is stated that within a year of the publication
of the book the price of opals declined fifty per cent in the European
market. Even if the superstition did not originate in either of these
ways, it was probably from a source quite as trivial, and it should
prevent no one from enjoying the pleasure to be derived from the
beauties of this gem.
Chemically,
opal is oxide of silicon, with varying amounts of water, the variation
being from 3 to 9 per cent. It is, therefore, closely allied to quartz,
but differs physically in being softer and not as heavy. Further, it
never crystallizes, and is soluble in caustic potash, which quartz is
not. It is infusible, but cracks and becomes opaque before the
blowpipe. In sulphuric acid it turns black, on account probably of the
organic matter it contains.
Its
hardness is sometimes as low as 5.5, though generally 6. Its specific
gravity is from 1.9 to 2.3. On account of its relative softness a cut
opal often does not retain its polish well, and requires frequent
smoothing. Opals, when first taken from the ground, are often softer
even than the above, and for this reason it is usual and desirable to
allow them to harden, or " season," as it is called, for some time
after quarrying before they are polished.
Opal,
as a mineral, is quite common, so that no one need suppose because he
has specimens labeled " opal" in his collection that he has as many
precious stones. It occurs in many varieties; and, especially if it
contains foreign matter, in many colors. Nearly all silica deposited by
hot waters is in the form of opal, so that the geysers of Yellowstone
Park build up cones of opal and fall into opal basins. This particular
form of opal is known as geyserite, and it is often differently colored
by different ingredients.
Wood
is often preserved by silica in the form of opal, the siliceous waters
taking away the wood and replacing it by opal, grain by grain, with
such delicacy and accuracy that the structure of the wood is perfectly
maintained. The minute shells which diatoms make consist of opal, and
when these dead shells accumulate to form deposits of some extent we
call the powdery substance tripoli, and use it for polishing silverware
and other metals. Other varieties of opal include hyalite, a variety
looking like transfixed water, so clear and colorless is it;
hydrophane, a translucent variety which sticks to the tongue and
becomes nearly or quite transparent when soaked in water; cacholong, a
porcelain-like variety; and menilite, a concretionary variety.
Common
opal varies from transparent to opaque, being most often translucent,
and sometimes exhibiting the peculiar milkiness of color
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