Uses
It
has been used for ornamental purposes, for mosaics, vases, inlaid work
and other ornamental objects. When used for jewellery it is cut en
cabochon. Formerly it was also used for the pigment,' ultramarine'. The
paint is now made artificially.
Occurrence
The
best quality of lapis lazuli comes from Badakshan in the N.E. corner of
Afghanistan, where ruby spinels also occur. It is also obtained from
Siberia, Chile and California. The rock has been produced by the baking
of an impure limestone near its contact with an intruded mass of
granite, when the contact has the right content of clay matter, sodium
and sulphur. It is obtained at Badakshan, northeast corner of
Afghanistan and the Mogok Stone Tract, Burma at Dattaw near Mogok,
where ruby and spinel are also obtained.
Chapter XXV
OTHER SEMI-PRECIOUS STONES
Introduction
MOST of these are not found in India and are only of mineralogical interest:—
1. Anatase.—When
brown and transparent it is cut for gem purposes. It is also known as
octahedrite. Tetragonal. Hardness = 5-1/2 to 6, specific gravity = 3.8
to 4.0. Colour: brown to black, adamantine lustre. Refractive index: w =
2.55, e = 2.49, double refraction, strong—0-06. Uniaxial and optically
negative. It occurs in Cornwall, England, France, Germany, Brazil, etc.
2. Andalusite.—Occurs
in orthorhombic crystals. Hardness = 7 to 7-1/2. Specific gravity =
3.1 to 3.2. Colours vary from grey, green, pink, red and violet.
Lustre: Vitreous to dull, transparent to opaque. Mean refractive index
154
