The precious or noble opal, fire opal, common opal, hydrophane, and
cachelong are different varieties of a mineral that is composed of
about nine parts silica and one part water.
The colors vary from chalky-white to bluish-white, from yellow to red,
and from a slight play of colors to the beautiful mingling of green,
blue, and red with the most remarkable kaleidoscopic effects.
The opal is 5.5 to 6 in hardness, specific gravity 2 to 2.1, lustre
glassy, and translucent from a slight to a very high degree.
The opal is found in an amorphous state and never crystallizes; in fact
from the condition of the pockets in which this mineral is found, the
indications are that the substance was once a fluid.
Under the blow-pipe the opal loses its translucency and cracks but does
not melt. Sulphuric acid will cause it to turn black, and in a cold
solution of caustic potash the opal is almost entirely soluble.
The precious or noble opal is found chiefly in the mines of
Czernowitza, between Kaschau and Eperies, in Hungary, and in Gracias á
Dios, a province in Honduras.
In olden times, the Greek and Turkish merchants carried opals from
Hungary to the Orient, and then they were shipped to Holland and sold
in Europe as Oriental opals.
The fire opal is of a yellowish-red color, and is found chiefly
in Mexico, although it also occurs in Hungary, the Faroe Islands,
Honduras, and Guatemala.
The common opal is found in Ireland, Denmark, Frankfurt, Guatemala,
and South Australia, and also in Hungary and Mexico. These opals are
translucent without fire or reflection.
The hydrophane is an opal that has lost color and brilliancy by reason
of the evaporation of its water. If placed in water or alcohol, this
stone becomes transparent, only to lose this quality when the water or
alcohol has evaporated.
The hydrophane becomes transparent more quickly in warm than in cold
water, but most rapidly in alcohol. If boiled in oil, the hydrophane is
said to retain its brilliancy for years.
The cachelong is milky-white, and nearly opaque, and is found in small
masses in the river Cach, in Bucharia, and also in Iceland.
Although one of the most magnificent of the gem stones, the opal for
many years was under the ban of superstition. Now, this splendid stone
once more commands a foremost place in the jewelers’ art, and the opal
mines of Hungary and Queensland are being worked to their fullest
extent to supply the demand.