Gems Notes and Extracts Augusto Castellani, Mrs. John Brogden 1871

216 GEMS.The Oriental topaz is a clear,coloured corundum, ofa very bright golden-yellow colour, having a specificgravity of 4. It has double refraction, although ina small degree. It is found in crystals, which aregenerally round, like those of other corundums, butsometimes it shows its primitive form, which is aquadrangular prism.Its hardness equals that of other corundums. It isfound atIndia.Peru, Ceylon, and in various other parts ofWith respect to value, it never equals the ruby,sapphire, and emerald. Sometimes it contains littlegrains which shine like those of the aventurine, butthese rather diminish than increase its value.There is a beautiful topaz in the mineralogical cabinetof Paris 0'023 metres long, and O014 metres wide.The Western topaz is divided into four differentkinds, viz., Brazilian, Saxon, Mexican, and Siberian.The Brazilian topaz, which is now most valued nextto the Oriental, is generally of a fine, clear deep yellow ;it has such a particular appearance that it is easilyrecognizable.One of its physical qualities, by which its naturemay even better be determined is, that when somewhatheated it acquires electricity, which it preserves sometimesfor thirty hours.It cuts rock crystal deeply, and its specific weightis 3-52. The most extraordinary of its peculiarities isthatof changing colour from yellow to a pink shadewhen warmed to a certain temperature.This discovery