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=
1-64. Strongly pleochroic. At times the cut stones resemble tourmaline
and apatite of similar colour. Chiasto-lite has carbonaceous
inclusions, and when cut in sections shows a black cross. Composition It is cut in
the step fashion. It is obtained from Spain, Ceylon and Brazil.
3. Apatite.—Apatite forms good hexagonal crystals and makes attractive gems. Hardness = 5 and soft. Specific gravity = ~ ~ "
It is usually coloured by impurities and colour varies from green to
pink, blue, purple or violet. Gem varieties are transparent. Mean
refractive index = double refraction = ,
uniaxial and negative. It is found in pegmatite, igneous and
metamor-phic rocks. Slue crystals are obtained from the Ruby Mines of
Surma. A deep sea-green variety is obtained from Devada, Vizagapatam
and from Kodur from the same district. A green apatite occurs in
pegmatite in Ajmere in Rajputana. Apatite of gem quality also occurs in
Germany, Switzerland and Ceylon.
4. Azurite and malachite.—These
are coloured blue and green and find use in decoration and
miscellaneous gems, as in pins, shirt buttons, etc. Monoclinic. Azurite
is often crystallized, but malachite occurs in rounded masses with a
fibrous or banded structure. Hardness = Specific gravity = .
Malachite—emerald green, grass-green; azurite—light blue to deep blue.
Translucent to opaque. Lustre—silky, vitreous or adamantine; mean
refractive index for malachite = and for azurite = Double refraction strong for malachite and 0-11 for azurite. Both are biaxial. Azurite is optically positive and malachite negative. Composition: Azurite CuOH); malachite CuC03.Cu(OH)2. Occurrence: Urals, Rhodesia, Chile, Belgian Congo and Australia.
5. Benitoite.—It
was discovered in 1907 in San Benito County, California, the only known
locality. Due to its deep blue colour, it resembles sapphire, but
differs by its
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