the
proverb says, only the diamond is able to "cut diamond." It is ranked
10 in the scale of hardness, corundum being the next below it. It is
really separated by a wide gap from the latter mineral, however, and
its hardness is as much greater than that of corundum as that of
corundum is greater than that of the first mineral in the scale. This
hardness of diamond affords a ready means of identifying it, as it will
scratch all other substances. It is popularly supposed that diamond is
the only mineral which will scratch glass to any extent, and a stone
found is often reported to be diamond because it will do this. As a
matter of fact, however, all quartz will scratch glass, and the harder
minerals, garnet, topaz, beryl, and others will do so easily. Minerals
which will scratch glass are, therefore, common. The diamond cuts glass
instead of scratchÂing it, and is the only mineral that will do this.
Although the diamond is so hard, it is not tough, and can be easily
broken with the blow of a hammer. It was a tradition of the ancients
that if a diamond were put upon an anvil and struck with a hammer, both
hammer and anvil would be shattered without injuring the diamond in the
least. One occasionally hears this statement made even at the present
day. It is entirely untrue, however, the diamond being as brittle as at
least the average of crystallized minerals.
The
specific gravity of the diamond is about three and one-half times that
of water, determinations showing variations between 3.49 and 3.53.
Carbonado is lower, ranging between 3.14 and 3.41. Diamond is thus a
comparatively heavy mineral, the only ones among the gems which much
exceed it in specific weight being hyacinth, garnet, ruby, sapphire,
and chrysoberyl.
Diamond
becomes strongly electric on friction, so that it will pick up pieces
of paper and other light substances. It does not retain its electricity
long, however, usually not over half an hour. It is not a conductor of
electricity, differing in this respect from graphite, which is a good
conductor. Diamond becomes phosphorescent on rubÂbing with a cloth,
giving out a light which is visible in the dark. Some stones, as if
they took up light from the sun and gave it out again, emit a
phosphorescent light after being exposed to the sun's rays for a time.
This has often been stated to be a property of all diamonds, but this
is not true, only certain stones exhibiting it. As first suggested by
Kunz, it is probable that this phosphorescence is due to minute
quantities of hydrocarbons which are heated by the friction given the
stone. It is curious to note that the light is in some cases given out
only from certain crystal faces. Thus
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