PRECIOUS STONES 11
color
necessary, and are valuable only for their hardness. These are used for
mechanical purposes. Of many other stones, the greater quantity found
is useless for any purpose.
The
stones accounted precious are the diamond, ruby, pearl, sapphire,
emerald, Oriental cat's-eye, opal, turquoise, alexandrite, and spinel.
Others
lying on the border-land between precious and semi-precious, and, in
fact, more valuable for natural qualities and as merchandise than some
specimens of those rated precious, are the andalusite, aquamarine,
golden beryl, hid-denite, olivine, tourmaline, zircon, and the finer
varieties of amethyst, topaz, and garnet.
In
order to express in definite terms the relative hardness of the various
stones, a scale was devised by a German mineralogist named Moh, in
numbers ranging from 1 to 10, the greater number representing the
hardness of the diamond, that being the hardest substance known.
It
should be understood, however, that these numbers attached to the
various stones do not designate absolute degrees of hardness, but are
approximate. All stones vary slightly. Some forms of diamond,
carbonado, for instance, are harder than others; some of the same form
are harder. The black diamonds of Borneo are harder than {he
Australian; the Australian are harder than those of India and Brazil,
and these, again, are harder than the African; and there are knots in
the grain of some specimens harder than the other parts of the stone.
But
as these variations are slight, and no other stone at its hardest
approaches the softest diamond, its hardness is always expressed by the
number 10.
The known variations of the ruby are quoted as 8.8 to 9 in the scale; the garnet ranges in its many varieties from 6 to 8.
The readings of specific gravity and the chemical composition of stones should be made with a like understanding.
