78 PRECIOUS STONES
sometimes
in the same stone. Diamond-cutters find knots in the grain of some
stones, which are as troublesome in the cutting as are knots in wood to
the sawyer. It is also said that the diamonds of Australia are harder
than others, as those of Africa are softer than those of India and
Brazil. Black diamonds are occasionally found in Borneo, however, so
hard that ordinary diamond-dust makes no impression on them; they can
only be ground by using their own dust.
Some of the diamonds found in South Africa reach the usual hardness only by degrees after exposure to the air.
It
has been found, in cutting, that diamond-powder from the surface of the
crystals has a greater cutting power than that obtained by the
pulverization of larger pieces, showing that the outside of the diamond
is harder than the inside.
The surface differs also, for a crystal often cuts easier in one direction than another, and some facets cut easier than others.
The supreme test of a diamond is its hardness. If a stone resists the ruby or sapphire, it must be a diamond.
"
Adamantine" is the term given to the character of lustre possessed by
the diamond. It is indescribable, but has a peculiar, fascinating
glitter which is immediately recognized by any eye which has once
looked upon its intense brilliancy.
The
refractive power of the diamond is very great. When a ray of light
enters a diamond, it is turned from its path to a greater degree than
by other transparent mediums, and its magnifying power is much greater
than that of glass. It also possesses in an extraordinary degree the
power to reflect and disperse the rays of light, producing thereby that
play of colors which comes from breaking up the ray of white light into
prismatic tints. This is one of its most admirable features, and is
evidence of good quality and skilful cutting.
Diamonds
are found as crystals more or less perfect, belonging to the group
known as the cubic or isometric system. The regular octahedron and the
rhombic dodecahedron (see
