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PRECIOUS STONES 175
fore
or after heating. It is found in Nova Scotia, New Jersey, and the Lake
Superior copper region. Used occasionally for the letter " N" in
initial jewelry.
Octahedrite
is a variety of rutile, probably the same, except for color, as that
called anatase. It occurs in flat glassy crystals of yellow and blue,
small but brilliant, in the United States and Brazil.
Pectolite
is a mineral of crypto-crystalline structure, having very fine
inseparable fibres, curved, radial, and interwoven. Hardness, 7, and
very tough; specific gravity, 2.8 to 3, the dark varieties being
heavier. It has no cleavage, and the fracture is uneven. Lustre, silky.
Composition varies, but averages, silica, 55 ; lime, 30 to 34; soda, 7
to 9; water, 3 to 5. Fuses to a white transparent glass. Exposed to the
weather, its china-like transparency changes to an earthy appearance,
which shows its fibrous structure plainly. Color, from white to pale
and dark green. Found in Alaska and California. A silicate of alumina
found in the Tyrols and Lake Superior region, is rated for hardness 4
to 5.
Phenacite
is found in Russia, Alsace, and Mexico. Its crystals and cleavage are
rhombohedral. Hardness, 7.5 to 8; specific gravity, 2.97 to 3. Lustre
vitreous; transparent to semitranslucent. Doubly refractive.
Composition: silica, 54; glucina, 46. It is unaltered before the
blow-pipe. Color, reddish yellow, brown, and colorless. When colorless
it is very brilliant under artificial light.
Prehnite
is an oily-green mineral resembling chrysoprase in lustre and color. It
occurs in right rhombic prisms, usually barrel-shaped and six-sided;
often reniform and botryoidal. Hardness, 6 to 6.5. Specific gravity 2.8
to 2.96. Lustre vitreous. Subtransparent to translucent. Composition:
silica, 43 ; alumina, 23.25 ; protoxide of iron and manganese, 2.25;
lime, 26; water, 4. Melts to a light green slag under the blow-pipe,
and dissolves slowly in muriatic acid. It is found in the Eastern
States and Lake Superior region.
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