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Another
test for topaz is to heat the powdered mineral with cobalt nitrate,
when it assumes a fine blue color, due to the alumina which it contains.
The
crystals of topaz belong to the orthorhombic system of
crystallization. They are usually elongated in the direction of the
prism, and have sharp, bright faces. They vary much in size, and often
are large. One crystal weighing twenty-five pounds was found in
Siberia. Large gems of topaz are, therefore, quite easily obtained.
Perhaps the largest cut topaz known was recently presented to Pope Leo
on the occasion
of his silver jubilee (1902). This stone weighed nearly four pounds. It was obtained originally in Brazil.
A
well-marked characteristic of all topaz crystals is their tendency to
cleave across the prism parallel with its base. Such a cleavage plane
can be seen cutting across the crystal shown in the upper right-hand
corner of the accompanying plate. This cleavage is so marked, and the
cleavage plane so bright and flat, that in cutting topaz for a gem a
cleavage surface is used as the upper face of the gem, and the other
faces formed around it. Owing to this easy cleavage the owner of a cut
topaz should be careful not to let the stone drop, as it might be thus
cracked or broken. Topaz takes a high polish, and colorless gems of the
mineral resemble the diamond considerably. They are, however, softer,
and have weak double refractive and dispersive power.
The name topaz is derived from the Greek name Topazios, which
is that of an island in the Red Sea. The gem known to the ancients as
topaz, however, was probably not our topaz, but chrysolite. Topaz
usually occurs in gneiss, or granite, with tourmaline, mica, beryl,
etc. In Brazil it occurs in a talcose rock, or in mica slate. It is
sometimes in sufficient abundance to form an essential rock
constituent. When so occurring, however, it has not the transparent gem
quality, but is white and opaque. Much of the Brazilian topaz occurs as
rolled pebbles, one of which is shown in the accompanying plate. These
occur in the beds of streams, having been left behind, owing to their
superior hardness, after the rock in which they were formed has been
washed away. When colorless they are known in the region as pingos d'agua (drops of water). The Portuguese call them "slaves' diamonds." A stone in the crown of Portugal, reputed to be a diamond
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