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PRECIOUS STONES.
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and
callainite and known as lazulite. It is essentially a hydrated
phosphate, but contains besides aluminium, a considerable proportion
of magnesium and iron ; copper is generally absent. It is softer and
rather heavier than turquoise, and is far more difficult to dissolve in
acids than that species. In colour it varies from a fine sky-blue to a
tint not far removed from that of pale lapis-lazuli—a perfectly
distinct mineral. Lazulite was occasionally used in ancient times as an
inlay, for instance in the gold armlet from the Oxus now preserved in
the British Museum.
Topaz.
Although
the topaz is a perfectly definite and distinct mineral species, yet
three different stones are commonly called by this name. But the topaz
known as " oriental topaz " is in reality the yellow sapphire, a kind
of corundum ; the occidental or Scotch topaz is nothing but yellow
quartz ; while the true topaz, sometimes spoken of as the Brazilian
topaz, is the only one which in reality may properly bear the name. The
hardness and specific gravity of these three stones are very different,
and furnish good criteria for their discrimination :
The
true topaz belongs to the orthorhornbic system ; its crystals are
prisms, usually having but one end regularly terminated. The cleavage
of topaz is highly perfect and basal, that is, transverse to the
length of the prism. The prismatic faces are commonly deeply channelled
but brilliant. The refractive indices of topaz, in the three directions
of the axes, are, for the yellow ray in a colourless crystal :
The double refraction of topaz is strong, and the pleochroism of coloured specimens very marked. A wine-yellow crystal from
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