76 PRECIOUS STONES.
them
to crack. Again, their lustre may be described as vitreous or glassy,
whereas in genuine pearls the lustre is soft, indeed almost dull.
Corning
now to the question of real stones, some methods of " faking " them, by
altering or improving their colour, might with advantage be briefly
touched upon before proceeding to the more interesting subject of
making them artificially.
The
colour of some stones may be altered by heat, and the process of "
burning " is one that was in use by the ancient Egyptians. Pliny also
refers to it in his time, and the method practised then is the same as
that in use to-day. It consists in either wrapping the stone up in
tinder and setting fire to the tinder, or packing the stone in very
fine powder, such as rotten stone or earth, and baking in a clay
crucible. Precautions have to be taken not to heat too highly, or the
stone may be hopelessly fissured. The yellow Brazilian Topaz assumes a
beautiful rose-red colour, and is then known as Burnt Topaz. The colour
is discharged from blue Sapphires by burning, and they become clear.
Bubies are quite unaffected, and retain their original colour.
Carnelians may be intensified in colour and much improved. Stones are
coloured also by soaking in chemical solutions. There are many
different methods. Perhaps the principal one in use is that for the
production of black-and-white Onyx. An ordinary Onyx consists of
alternate layers of blue Chalcedony (a crypto-crystalline form of
silica) and white opaque Cachalong, another variety of silica. The
Chalcedony layers are slightly porous, and if soaked in oil for some
time absorb a certain amount ; this is blackened by de-hydrating with
strong sulphuric acid. Small specimens
