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PRECIOUS STONES
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159-
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SPHENE
The name is from the Greek sphen, a
wedge, the shape of the crystals. The dark varieties of this stone were
formerly called titanite, and the lighter only, sphene. It is found in
Switzerland, Norway, the Urals, Finland, Wales, Ireland, Germany,
Tyrol, and North America.
Crystallization monoclinic, occurring in wedge-shaped crystals; occasionally massive.
Hardness, 5 to 5.5; specific gravity, 3.25 to 3.5.
Lustre adamantine to resinous; transparent to opaque.
Cleavage in one direction, parallel to faces of the prism, sometimes perfect: fracture conchoidal, imperfect.
Doubly refractive (index, 1.88); dichroic; brilliant dispersive powers when transparent; electric in the polished state.
Composition: titanic oxide, 41; silica, 31; lime, 27; iron oxide, 1.
Before
the blow-pipe the yellow varieties do not alter in color, but others
become yellow; on charcoal they fuse on the edges to dark glass.
Color, pale yellow to green and brown, showing mixed colors; streak uncolored.
Cut step and brilliant.
.
Some
varieties of this stone are opaque; others exhibit various degrees of
transparency; few are sufficiently clear* to be used as gem stones, and
its softness prevents these from being generally used as jewels.
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