by
few other minerals. In reference to its occurrence in the diamond it is
known as the adamantine luster. It combines the peculiarity of an oily
luster with that of glass and that of a metal. It is doubtless due to
the high refractive power of the mineral, which causes more than the
ordinary number of rays of light to come to the eye. In the impure
forms of diamond the greasy or oily luster becomes more pronounced.
Once the eye becomes accustomed to the peculiar luster of diamond the
stone may easily be distinguished by it from glass or minerals with a
vitreous luster, such as quartz. Certain other minerals, however, such
as cerussite, zircon, and to some extent sphene, exhibit the adamantine
luster. In the glass known as strass, used to make imitation diamonds,
the adamantine luster is well reproduced.
Diamond
is usually transparent, but it may be translucent, and even opaque,
especially the black varieties. Even otherwise transparent diamond
often contains inclusions which cloud and interrupt its clearness.
These constitute the "flaws" which so often injure the value of a
diamond and prevent it from being of the " first water." These
inclusions may be simply small cavities, sometimes so numerous as to
make the stone nearly black; or they may be particles of other
minerals, such as chlorite, hematite, or carbonaceous matter. If the
latter, the flaws can sometimes be burned out by careful heating.
As
already remarked, the refractive power of the diamond is very high. The
rays of light entering it are bent at a high angle, causing a large
degree of what is called total reflection within the stone. The effect
of this is to light the stone's interior. Moreover, the rays of light
are concentrated on a smaller part of the surface than is the case with
less highly refracting minerals, and thus also internal illumination
is produced. The most important result of the high refractive power of
the diamond is the wide dispersion of the spectrum, causing the red
rays to be widely separated from the blue rays, and strong lights of
one color to be transmitted to the eye, as could not be the case were
the different rays less widely separated. It is this power of flashing
different colored lights which gives the diamond one of its chief
charms. The index of refraction ranges from 2.40 for the red rays to
2.46 for the violet rays. Ordinary glass has an index of refraction
for the red rays of only 1.52, and for the violet 1.54, making the
spectrum only about half as long as that produced by the diamond.
Another
pleasing property of the diamond is the fact that it is usually more
brilliant by artificial light than by natural, although some individual
stones have a reverse behavior.
Diamond is much the hardest substance known in nature, and as
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