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 pseudo-morph (see Chapter IV., " Cleavage.") In colour
it is blue or greenish-blue, sometimes opaque, varying between that
and feeble trans-lucency, though it should be said that in all forms,
even those considered opaque, a thin cutting of the stone
Flickr
Stumbleupon
Scribd
