THE TURQUOISE 105
appears
almost transparent, so that the usual classing of it among the opaque
stones must be done with this reservation. In composition it contains
about 20 per cent, of water, about a third of its substance being
phosphoric acid, or phosphorus-pentoxide ; sometimes nearly half of it
is alumina, with small quantities of iron in the form of variously
coloured oxides, with oxide of manganese. The great proportion of
water, which it seems to take up during formation, is mostly obtained
in the cavities of weathered and moisture-decomposing rocks. Its
average formula may be said to be
and some-
times .
It must therefore follow
that
when the stone is heated, this water will separate and be given off in
steam, which is found to be the case. The water comes off rapidly, the
colour of the stone altering meanwhile from its blue or blue-green to
brown. If the heat is continued sufficiently long, this brown will
deepen to black, while the flame is turned green. This is one of the
tests for turquoise, but as the stone is destroyed in the process, the
experiment should be made on a splinter from it.
This
stone is of very ancient origin, and many old turquoise deposits, now
empty, have been discovered in various places. History records a
magnificent turquoise being offered in Russia for about £800 a few
centuries ago, which is a very high price for these comparatively
common stones.
Owing
to the presence of phosphorus in bones, it is not uncommon to find, in
certain caves which have been the resort of wild animals, or into which
animals have fallen, that bones in time become subjected to the oozing
