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Ch. 4: Topaz

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128
PRECIOUS STONES.
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.
Ch. 4: Topaz Page of 296 Ch. 4: Topaz
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