and
quite opaque. It occurs in sandstone of very old formation, and is
found in Bahia, and of late in Mexico. It has no utility when cut, but
reduced to powder it is used for polishing diamonds and other gems, and
is especially prized by the watchmakers of Switzerland. Its hardness is
identical with that of the crystallized diamond ; its specific gravity
is 3.012 to 3.016. It is not used to so good advantage in proportion to
its weight as "boart." It is known in commerce under the name of carbonate, or carbonic diamond.
Crystallized diamonds in their natural state are called "rough diamonds."
The diamond is always sold by weight. The standard of weight for all precious stones is the carat; a
name derived, it is said, from the seeds of a pod-bearing plant used in
the East to measure gold dust. The carat is 4 grains; that is, diamond
grains, which differ slightly from troy grains, as it takes five of the
former to weigh four of the latter; or more exactly, one carat = 3*174
gr. troy.
The
carat is universally employed in the commerce of jewelry, but it is
not rigorously the same in all countries. The following are the weights
of the different carats in milligrammes:—
