The diamond is one of the most precious minerals, and yet it consists
of pure carbon, the most common substance that is known, a substance
that is present in all animal and vegetable bodies and in the larger
number of minerals. When carbon is crystallized the result is the
diamond, which is always found in detached crystals, either octahedrons
or rhombic dodecahedrons, the planes of the angles being often convex
or rounded,—this curving crystal being peculiar to the diamond.
The cleavage is perfect, and, parallel to the faces of the octahedron,
the fracture is conchoidal or curved. The diamond is not acted upon by
acids or alkalies, is infusible but combustible, and burns under heat
of a very high temperature. Diamond powder burns readily, but larger
pieces are not affected by the blow-pipe.
The diamond is a non-conductor of electricity, but acquires positive
electricity when rubbed, and retains it for half an hour. After
being exposed to the solar rays, the diamond presents a distinct
phosphorescence in the dark. It possesses single refraction, but
belongs to those bodies which reflect light most strongly, and its
magnifying power is much greater than that of glass; it does not
polarize light; its lustre is adamantine, and specific gravity 3.5 to
3.6. The diamond is the hardest of all known minerals, ranking No. 10
in Moh’s scale of hardness.
White, and the different shades from very light yellow to dark yellow
or canary, comprise, according to the popular idea, the colors of the
diamond. Yet the diamond is found in green, red, blue, brown, olive,
orange, and black, and also in the various shadings of these colors and
in opalescent tints.
As the limpid or white diamond surpasses all other white stones in the
power of its lustre and the magnificence of its fire, so do the colored
diamonds outrank the emerald, ruby, sapphire, and other gems of like
colors.
Colored diamonds, excepting light yellow and brown, are rare, and hence
are the most valuable of precious stones. The limpid or perfectly white
and the white with a bluish tint are the most sought after, while fine
deep golden yellow or canaries and pronounced fancy colors always find
a ready market.
Diamonds come principally from the mines in South Africa; some are
found in Brazil and India, and fewer in Sumatra, Borneo, the Ural
Mountains, and Australia. Crystals have also been found in the United
States.
The amorphous or carbon diamond is found only in Brazil. The pebbles or
masses are opaque, steel-gray to black in color, and sometimes weigh
1,000 carats.
This carbonate is principally used to point rock-drills and for other
engineering purposes. The coarse variety of crystallized diamonds
is called bort, and as this is unfitted for gem purposes because of
imperfections, it is ground into powder and used for cutting and
drilling precious stones.
White sapphires, white zircons, white topaz, and rock-crystal sometimes
pass for diamonds. The first two are heavier, the topaz lacks
brilliancy, and the crystal is lighter than the diamond.
It is also the case that these four stones, especially the crystal, are
easily scratched by a diamond.
The best style of cutting for a diamond is the brilliant, of 66 facets,
including the table and culet. The proper proportions of a well cut
brilliant is ⅓ for the crown and ⅔ for the culet. The table and culet
must also be in proportion to the size of the stone.