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PRECIOUS STONES
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MOONSTONE
(Adularia)
An opalescent
variety of feldspar, so named from a supposed chatoyant similarity to
the moon. It is found in Ceylon, Mount Adula, and occasionally in
Northern Europe and the United States.
Crystallization monoclinic; usually in thick, rectangular prisms or crystalline fragments. Also massive and granular.
Hardness, 6 to 6.5; specific gravity, 2.4 to 2.6.
Lustre
pearly and chatoyant; transparent to subtrans-lucent. If cut with a
high dome, an effect is sometimes produced resembling the cat's-eye.
Cleavage perfect parallel to the shorter diagonal; fracture conchoidal or uneven.
Doubly refractive (index, 1.55); dichroic; electric when polished.
It consists of silica, 64.7; alumina, 18.4; potash, 16.9.
It is not attacked by acids; fuses only on the edges, but with borax melts slowly to transparent glass.
Color,
light tints of pearly gray, blue, green, and flesh-red. Sometimes the
play of light shows one of these tints floating on a gray background.
It is cut en cabochon or rounded on either side.
Adularia is the same stone when white or colorless sub-transparent.
Selenite,
one of the finer varieties of gypsum, is found in some of the central
counties of England. Its name signifies " moonstone," but it is not
the mineral known to jewellers as moonstone. It is soft, has a
moonshine lustre, and is used for beads, etc.
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