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LAPIS LAZULI
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This
stone was the sapphire of the Greeks, Romans, and Hebrew Scriptures.
Pliny likened it to the blue sky adorned with stars. Large quantities
of worked pieces of it are found in early Egyptian tombs, and the
Chinese have long held it in high esteem. Marco Polo visited Asiatic
mines of the mineral in 1271 A. D., and these had doubtless been worked
for a long time previous. Besides its value as a stone, it was in
former times used as a blue pigment, giving the ultramarine-blue. In
modern times not only has the esteem in which the stone is held for
ornamental purposes declined, but the mineral can be artificially made
so as to give the desired blue color for paints, and thus the use of
the natural lapis lazuli has greatly diminished. It is still, however,
carved to make vases, small dishes, brooches, and ring stones, and is
used to a considerable extent for mosaic work. When, also, pieces of
sufficient size and of a uniform color can be found, large carved
objects may be made which command a high price.
The
stone known as lapis lazuli as it occurs in nature is not a single
mineral but a mixture of several, among which are calcite, pyrite, and
pyroxene. From these, however, it is possible to separate a mineral of
uniform composition sometimes crystallized in dodecahedrons, which is
probably the essential ingredient of the stone. This mineral is known
as lazulite, and in composition is a silicate of soda and alumina, with
a small quantity of sodium sulphide. It is by making a substance of
this composition that the artificial ultramarine is produced. The
artificial is said to be as good as the natural for a pigment, and can
be produced for a three-hundredth part of the cost. The natural "lapis
lazuli" has a hardness of 5-1/2 and a specific gravity about like that
of quartz. It is quite opaque. In color it is blue, varying from the
prized ultramarine to paler, and at times is of a greenish shade. It is
said the pale colored portions can be turned darker by heating to a red
heat. When the variety from Chile is heated in the dark it emits a
phosphorescent green light. The stone in nature is often flecked with
white calcite. Portions so affected are not considered as valuable as
the uniform blue. Grains of pyrite are also usually scattered through
the stone, giving the " starry " effect referred to by Pliny.
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