of
Arabia, from whence the Romans obtained a mineral whicli they called
topazos and topazion, which mineral to-day is termed chrysolite. The
mineral topaz is found in Cornwall and in the British Isles generally ;
also in Siberia, India, South America and many other localities, some
of the finest stones coming' from Saxony, Bohemia, and Brazil,
especially the last-named. The cleavage is perfect and parallel to the
basal plane. It crystallises in the 4th (rhombic) system; in lustre it
is vitreous ; it is transparent, or ranging from that to translucent;
the streak is white or colourless. Its colour varies very much —some
stones are straw-colour, some are grey, white, blue, green, and orange.
A very favourite colour is the pink, but in most cases this colour is
not natural to the stone, but is the result of " burning," or " pinking
" as the process is called technically, which process is to raise the
temperature of a yellow stone till the yellow tint turns to a pink of
the colour desired. The topaz is harder than quartz, as will be seen on
reference to the " Hardness" table, and is composed of a silicate of
aluminium, fluorine talcing the place of some of the oxygen. Its
composition averages 16'25 per cent, of silica, 55.75 per cent, of
alumina, or oxide of aluminium, and fluoride of silicium, 28 per cent.
Its formula is [Al (F, 0 H)], Si O4. or (AlF)2SiO4.
From this it will be understood that the fluorine will lie evolved when
the stone is fused. It is, however, very difficult to fuse, and alone
it is infusible under the blow-pipe, but with microcosmic salt it fuses
and evolves fluorine, and the glass of the tube in the open end of
which the stone is fixed is bitten with the gas.
