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Lazurite
is practically always of a blue colour, the commonest shade being an
azure blue, sometimes Berlin-blue, rarely of a violet tinge, sometimes
greenish blue. Darker parts, merging to an indigo blue, are seen, and
often white streaks and yellow metallic patches, due respectively to
admixtures of Calcite and Iron Pyrites. As with other precious stones,
the richer coloured kinds are known as " masculine," and the paler ones
as " feminine." It is translucent to opaque. The streak is ultramarine.
On heating to a dull red the pale blue shades are rather improved, for
they often turn to a richer and deeper blue colour. But sometimes the
effect of heating is to make the mineral greenish blue. At a higher
temperature the blue colour is completely discharged, and the mineral
finally fuses to a colourless glassy mass. The specific gravity varies
from 2"38 to 2'45, hence it is one of the lightest precious stones. The
fracture is uneven. The hardness is 5 to 5-1/2. The crystalline form is
cubic, but it is very rarely that crystals are seen. When they are
found they are small, and in the form of the dodecahedron, parallel to
the faces of which there is an imperfect cleavage. The substance known
as Lapis Lazuli is really a mass of fine particles of this mineral,
along with several others, in most cases embedded in Calcite. It is
nearly always found in limestones that have been subjected to extensive
meta-morphism, either by proximity to a great plutonic mass or by
dynamic changes. The chemical composition is complex, and by no means
invariable ; it may be expressed as essentially  —a sodium
aluminium silicate with sodium sulphide. It is of some interest to note that Epiphanius recommends the use of
p.s.
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