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Ch. 25: Other Semi-Precious Stones

Ch. 24: Lapis-Lazuli Page of 187 Ch. 25: Other Semi-Precious Stones Text size:minus plus Restore normal size   Mail page  Print this page
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
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Ch. 24: Lapis-Lazuli Page of 187 Ch. 25: Other Semi-Precious Stones
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