8 GEMS AND PRECIOUS STONES.
positive
and negative poles. Pyro-electricity is mostly developed by those
minerals whose crystals are terminated by different crystalline face3
at the different ends, and which are called from this peculiarity
hemimorphic.
Besides
tourmaline, topaz and axinite afford examples of pyro-electricity. Very
great care must be taken in heating stones by fire, because in some
cases where cleavage is very perfect they might split, as in topaz,
which has a very perfect cleavage.
An
easy method of distinguishing between green tourmaline and olivine is
by heating, the former developing pyro-electricity while the latter
does nor.
Cleavage.
This
is the property possessed by many minerals of splitting in certain
directions more readily than in others ; in each cleavable mineral
bearing a constant relation to a certain face, or to certain faces, of
the form in which the mineral crystallizes. Minerals may cleave in one
or more directions, but one cleavage is generally to be obtained with
greater ease than the others. Topaz has a very perfect basal
cleavage—that is, parallel to the base of the prism, or at right angles
to the longer axis, and in rolled stones a portion of a smooth cleaved
face is often found on topaz. Beryl also has a basal cleavage, although
somewhat indistinct. There is also a basal cleavage in corundum and its
varieties, which is sometimes perfect. The diamond has a perfect
cleavage, parallel to the faces of the octahedron, this being the
primary form of the cubical system, to which the diamond belongs.
Cleavages are known as, perfect, when smooth and readily obtained, or imperfect, when obtained with difficulty. The latter are not so smooth.
The
diamond cutter avails himself of this natural property of this gem to
remove damaged portions, &c. Dr. Wollaston, in the early part of
this century, was one of the first to call attention to the advantages
offered by the ready cleavage of the diamond. He purchased a rough,
badly-flawed stone from a firm who considered it too bad to pay for the
cutting; then, removing the defective parts by cleavage, he had the
perfect portion cut, and resold the cut stone to the firm from whom he
first bought it, at a very large profit.
But
the cleavage of the diamond must have been known long before this
period, because Dr. Boot, writing in 1G09, tells us that he knew a
physician who boasted that he could divide a diamond into small scales
like a piece of talc. The flat gems of Indian origin also point to the
fact that the cleavage of the diamond was well known.
In
early times an idea existed that owing to the extraordinary hardness of
the diamond it could not be broken by a hammer on an anvil; but this
was erroneous, because a diamond can be reduced to grains by a. heavy
pestle and mortar, on account of its perfect cleavage. This will,
perhaps, account for the fact that the ancients had no knowledge of
great diamonds, as they placed them upon the anvil to test their
genuineness.
This
supposed property of the diamond of resisting the blow of a hammer is
mentioned both by Lucretius and Pliny. The latter says that the test of
all these diamonds is made upon an anvil by blows of the hammer, and
their repulsion from iron is such that they make the hammer fly to
pieces, and sometimes the anvil is broken. This error was not
eradicated until comÂparatively modern times, for in 1476, when after
the battle of Morat, the Swiss soldiers seized upon the tent of Charles
the Bold, they found in it, among other treasures, a certain number of
diamonds, and in order to test whether they were genuine, struck them
with hammers and hatchets, and of course broke many of them to pieces.
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