Touchstone is a black, hard, rugged schist, of very fine close texture.
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.
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.