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Ch. 24: Lapis-Lazuli

Ch. 23: Turquois Page of 187 Ch. 25: Other Semi-Precious Stones Text size:minus plus Restore normal size   Mail page  Print this page
Varieties, Grades
(1)  Turquois matrix consists of bits of turquois in a limonite matrix.
(2)  Odontolite—it is not real turquois, but is bone fossil turquois. It is coloured blue either artificially or naturally distinguished by its organic structure.
OCCURRENCES
It is formed from solutions, its associate being limonite, quartz, felspar, etc. Good turquois comes from Persia and Egypt. In India it is found in the Ajmer hills and at Ramgarh, Rajputana.
Variscolite is another hydrated aluminium phosphate of an apple-green colour which is caused by the presence of chromium and vanadium. It is used at times in place of turquois.
Chapter XXIV
LAPIS LAZULI (LAZURITE)
Introduction
LAZURITE has been known from very ancient times, as it was used for mosaics, inlaid work, etc., and was also used for the pigment ultramarine. It is a mixture of a number of minerals, of which lazurite is the most im­portant, as it imparts the blue colour. Other minerals present are hauyne, diopside, amphibole, calcite and pyrite. Hardness = 5-1/2. Specific gravity varies from 2-5 to 2-95. Sp. gr. of the pure mineral is 2-4. Colour varies from deep blue to azure blue, etc. Lustre: Vitreous to greasy. Opaque to transparent. Cubic. Refractive index = 1-50. It has a complex chemical composition. It is a complex alumino-silicate and sulphide of sodium accompanied by HC1.
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Ch. 23: Turquois Page of 187 Ch. 25: Other Semi-Precious Stones
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