the
sapphirine quartz, the jasper, etc ; but except in the forms of cameos
and intaglios these stones are but little esteemed. Rich deep-coloured
amethysts, with the colour not quite uniformly distributed throughout
their substance, are perhaps the most prized form of quartz. The colour
of such amethysts gains in beauty from the dichroism of the stone and
from its peculiar rippled and parquetry structure. Such amethysts are
wrongly called oriental amethysts by jewellers (some do come
from India), for the true oriental amethyst is a purple sapphire, not
quartz at all, and an excessively rare stone.
The
localities for choice specimens of amethyst, sard, chrysoprase,
chalcedony, etc. are legion. The amethysts of Brazil and Ceylon, the
agates of Uruguay, the chrysoprases of Silesia, the cornelians of
Arabia, and the jaspers of Egypt are famous.
Not
only have the dark-coloured onyxes of commerce been artificially dyed
or stained, but a large proportion of agates, cornelians, sards, etc.
have been similarly altered. A moderate heat reddens many varieties of
quartz originally grey and brown, while a soaking in sugar or honey,
followed by treatment with strong sulphuric acid, brings out black and
white bands in the natural grey onyx.
Hydrochloric
acid develops a lemon-yellow colour in white chalcedony, while a strong
blue colour may be imparted by causing Prussian Blue to be precipitated
within the stone by alternate soaking in solutions of green vitriol and
of prussiate of potash.
Lapis-lazuli.
Lapis-lazuli
or azure stone is not a definite mineral but a mixture, in variable
proportions, of several minerals. Generally calcite forms the chief
part of the colourless patches in the stone, but there will be present
two or even three complex silicates possessing a beautiful blue colour.
One of these is known as Haiiyne or Haiiy-nite, another is true
ultramarine, and another is called sodalite. They are all silicates and
all contain much alumina' but soda is also
