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162
GEMS.
Petersburg, to whom it was dedicated by the scientific Hess, who first studied it and gave it a distinct place.
It is very similar to the green garnet, but harder. It is found at Bissersk, in Siberia.
LXXIV.
TOUCHSTONE.
Touchstone is a black, hard, rugged schist, of very fine close texture.
More than any other stone, it preserves marks of whatever metals are rubbed over it ; this property gained it the name of touchstone, or stone by which the qualities of the different precious metals may be tested.
This mineral substance, also called Lydian stone, is somewhat attracted by the magnetic needle, and dif­fuses a clayey odour when bathed in warm water. It takes a good polish, and is not hurt by acids. Its specific gravity is 2-415.
Although the touchstone is generally black, it is also found of an extremely dark green. It does not emit sparks under the flint, and is perfectly fusible without the addition of any other material, provided it be exposed to an intense fire, and then it is converted into a black or greenish glass. It is somewhat harder than horn stone properly so called-* it much resembles basalt, and when broken, its fragments are rhomboidal.
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