Colour
varies from sea-green to green or blue, resembling aquamarine. It
occurs in metamorphic rocks. It is found in Kashmir with sapphires,
also in Ural Mountains, and Minas Geraes, Brazil. Mean refractive index
= 1-65 and double refraction =0-02. Biaxial and optically negative.
12. Felspar.—It
is an important rock forming mineral, most of it is opaque and
valueless but some of it is of gem quality. Transparent and colourless
varieties are known as adularia and moonstone—opalescent. Moonstone
varieties of albite and adularia are also found, (a) Moonstones are
cut cabochon and are obtained in the gem gravels of Ceylon and in Burma
in pegmatite in the area east of Mogok. It is also locally cut at Mogok
and sold as cheap precious stones. It is also obtained from Switzerland
and Elba, {b) Amazon stone is a variety of microcline and has a
pleasing colour like that of jade and is cut cabochon. It is used in
brooches and pendants. It is obtained from Ural Mountains,
Pennsylvania, Virginia and Colorado. It was also obtained two miles
south of Dom-chanch, Hazaribagh district, Bihar and also in Kashmir,
and from near Saidapuram, Nellore, Madras, (c) Sunstone or aventurine
is oligoclase, recorded from Mogok by Prof. Judd. It is obtained in
Norway and Siberia, (d) Labra-dorite—soda lime felspar—has a
beautiful play of colours in blue and green and less often in yellow,
red and grey. The mineral takes its name from Labrador and also
occasionally found in the syenites of Mogok.
13. Flourite or flourspar.—It
occurs in good cubic crystals—twinning seen as interpenetration cubes.
Cleavage—octahedral, perfect. Hardness=4. Specific gravity =
. It has attractive colours and may be transparent to opaque. It has a
vitreous lustre and is fluorescent and phosphorescent. Isotropic.
Refractive index =
Composition: CaF2. It is carved as vases, dishes, etc. The best material is obtained from Derbyshire, Cumberland,157
