|
114
|
THE DIAMOND
|
||
|
|
|||
|
as
7.5 to 7.8. That means that its hardness is half way or more between
quartz and topaz, but the difference between 7 and 8 is not nearly as
great as between 9 and 10. It is said that the difference between 9 and
10 is greater than it is between 9 and 1. The scale therefore does not
represent exact and absolute degrees of hardness, but is an arrangement
of minerals of different degrees in that quality, numbered for
convenient reference.
Nor
is the diamond always of the same degree of hardness. Stones from wet
diggings are usually harder than those from dry diggings. African
diamonds are softer than Brazilians; Indian are harder, and those of
Borneo and Australia are said to be hardest of all. The " nyf " or skin
of a crystal is harder than the interior, and frequently there are
knots in the grain, so much harder that it is difficult to cut them.
Cut with or against the grain of a diamond, and the wheel makes little
impression; it must be cut across the grain. Sir William Crookes is
reported to have said of the Koh-i-noor, that in cutting one of the
facets near a yellow flaw, the crystal became so much harder the
further it was cut, that after working the mill for six hours at the
usual speed, little impression was made, and that the work proceeded
very slowly even when the speed was increased to 3,000 revolutions per
minute. Other portions of the stone were comparatively soft, but became
harder as the outside was cut away.
The
only rival of the diamond in hardness is the metal tantalum, of which
it is said that in the effort to bore a hole through a plate of it, a
diamond drill driven at the rate of 5,000 revolutions per minute for
three days and nights, made a depression 1/4 mm. deep.
|
|||
|
|
|||