This
stone is now very rare; and when to the fineness of its quality it
joins a soft clear colour with a satin-like lustre, it acquires a
considerable value. But the topaz, however perfect, never reaches the
price of a ruby or a sapphire, or even a fine emerald of equal
dimensions.
Occidental Topaz.—Stones
thus designated are not corundums. They are of a more complex
composition; and the analysis of specimens brought from different
localities proves that occidental topazes are not identical. They have
for a long time been divided into four varieties:—
It
should be remarked, however, that while the proportions of the elements
differ in each of these varieties, their nature is the same. The
occidental topaz is in all cases formed of alumina, silica, and fluoric
acid. The presence of this last substance, found in no other precious
stone, characterizes perfectly the genus topaz in respect to chemical
composition.
The primitive type of crystals of topaz is the right rhomboidal prism (Fig. 58).
Certain
modifications occur in the form of these crystals, which, joined to
their colour, enable a person to decide as to which of the varieties
any topaz in question belongs.
