2Al{2}O_{3},3SiO_{2}--is usually found in a crystalline or granular
form, and mostly in granite and in the interstices of the plates, or
laminae, of rocks called schist. One variety of this, which is a deep
hyacinth in colour, though often of a brown-tinted red, is called
"spessartine," or "spessartite," from the district in which it is
chiefly found, though its distribution is a fairly wide one.

_The Lapis-Lazuli._

The lapis-lazuli, sometimes called "azure stone," is almost always blue,
though often containing streaks of white and gold colour, the latter of
which are due to the presence of minute specks or veins of iron pyrites,
the former and colourless streaks being due to free lime, calcite, and
other substances which have become more or less blended with the blue
colour of the stone. It has a vitreous lustre, crystallises in the 1st,
or cubic system, and is a complex substance, varying considerably in its
ingredients in accordance with the locality in which it is found, its
matrix, and the general geological formation of the surrounding
substances, which may, by the penetration of moisture, be brought to
bear upon the stone, thus influencing to a great extent its chemical
composition. So that we find the stone composed of about a quarter of
its substance of alumina, or oxide of aluminium, silica to the extent of
almost half, the remainder being lime, soda, sulphur, and occasionally
traces of copper and iron. It is mostly found in granite and certain
crystalline limestone rocks, in fairly large masses. It is of great
antiquity, figuring extensively in ancient Egyptian history, both in its
form as a stone and ground up into a pigment for the decoration of
sacred and royal vessels and appointments. When so ground, it forms the
stable and magnificent colour, _genuine_ ultramarine, which is the
finest and purest blue on the artist's palette, but owing to its
extremely high price its use is not in very great demand, especially as
many excellent chemical substitutes of equal permanence are obtainable
at little cost.

_The Turquoise._

The turquoise is a pseudomorph (see Chapter IV., "Cleavage.") In colour
it is blue or greenish-blue, sometimes opaque, varying between that and
feeble translucency, though it should be said that in all forms, even
those considered opaque, a thin cutting of the stone appears almost
transparent, so that the usual classing of it among the opaque stones
must be done with this reservati