PRECIOUS STONES. 35
act
as a magnet; on cooling, the charges on the poles reverse, positive
becoming negative. If a crystal with such a charge be dusted with a
fine mixture of sulphur and red lead, the yellow sulphur will be
attracted to the portions charged with positive electricity, while the
red lead goes to the negatively charged portions.
The
pyroelectric behaviour of Tourmaline and Topaz may be made use of to
distinguish these minerals from others of similar colour.
Though
some minerals show magnetic properties, only one is of any importance
in the present case : this is Iserine, one of the titaniferous iron
ores, which has been used as an ornamental stone.
IV. Specific Gravity.
The
specific gravity of a substance is the ratio of the weight of a given
volume of that substance to the weight of an equal volume of a standard
substance. Water is always taken as the standard in dealing with
minerals. Specific gravity is of the greatest importance in dealing
with precious stones, as it affords a means of identifying many of
them when cut and polished, without in any way damaging the stone.
There
are many methods of determining the specific gravity of a substance;
three will be briefly described here applicable to the cases (a) where
there are many small fragments of the mineral available; (b) where it is desired to deal with a small cut stone, and (c) where
a relatively large specimen can be used. Of course the use of the
various methods is not thus restricted; they are merely cited as
examples.
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