The
Oriental topaz is a clear, coloured corundum, of a very bright
golden-yellow colour, having a specific gravity of 4. It has double
refraction, although in a small degree. It is found in crystals, which
are generally round, like those of other corundums, but sometimes it
shows its primitive form, which is a quadrangular prism.
Its hardness equals that of other corundums. It is found at Peru, Ceylon, and in various other parts of India.
With
respect to value, it never equals the ruby, sapphire, and emerald.
Sometimes it contains little grains which shine like those of the
aventurine, but these rather diminish than increase its value.
There is a beautiful topaz in the mineralogical cabinet of Paris 0.023 mètres long, and 0.014 mètres wide.
The Western topaz is divided into four different kinds, viz., Brazilian, Saxon, Mexican, and Siberian.
The
Brazilian topaz, which is now most valued next to the Oriental, is
generally of a fine, clear deep yellow ; it has such a particular
appearance that it is easily recognizable.
One
of its physical qualities, by which its nature may even better be
determined is, that when somewhat heated it acquires electricity, which
it preserves sometimes for thirty hours.
It
cuts rock crystal deeply, and its specific weight is 3-52. The most
extraordinary of its peculiarities is that of changing colour from
yellow to a pink shade when warmed to a certain temperature. This
discovery
