The
white topaz considerably exceeds rock crystal in lustre, and in Brazil
is called _mina nova_.[4]
[4] The pink topaz is made from the yellow, which, when of
intense colour, is put into the bowl of a tobacco pipe, or small
crucible, covered with ashes or sand: on the application of a
low degree of heat, it changes its colour from a yellow to a
beautiful pink. It contains fluoric acid, which may be the means
of this change.--_Mawe_.
_Why is ruby of such a brilliant colour?_
Because a sixth of it is chromic acid, while other gems, as the garnet,
are coloured by oxide of iron. The most esteemed, and at the same time,
rarest colour, of the oriental ruby, is pure carmine, or blood-red of
considerable intensity, forming, when well polished, a blaze of the most
exquisite and unrivalled tint. It is, however, more or less pale, and
mixed with blue in various proportions; hence it occurs rose-red and
reddish white, crimson, peach-blossom red, and lilac blue--the latter
variety being named oriental amethyst. A ruby perfect both in colour and
transparency, is much less common than a good diamond, and when of the
weight of three or four carats, is even more valuable than that gem.
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