possess
little or no cleavage, thus increasing their value as abrasives and for
setting in drills, saws, etc. The true bort occurs as rounded forms
made up of a confused aggregate of crystals, and is harder than
ordinary diamond. Fragments of crystals of no value as gems, or any
crude diamond dust, are also known as bort in trade. Carbonado is a
name given to black diamond, which has more or less crystalline
structure. This graduates into the crystallized mineral. Either of
these is more valuable than the crystallized diamond for industrial
purposes, although of no value as gems.
Usually
the diamond is colorless or white, although shades of yellow are also
common. It is also known in shades of red, green, and blue, and in
brown and black. The two latter are rarely transparent, and grade into
bort and carbonado.
About
half the diamonds found are tinged to some degree. If the color is but
slight, the stone is considered less valuable than if perfectly
colorless; but a diamond of pronounced color is the most valuable gem
known.
Among
colors of diamonds, blue is the rarest. The largest and most valuable
colored diamond known is the Hope Blue, weighing 44-1/2 carats. This is
valued at about one hundred thousand dollars. It has a brilliant deep
blue color and is without a flaw. A deep blue diamond, weighing 67-1/8
carats, was long worn in the French crown, but it was stolen in 1792
and has never been recovered. Red diamonds vary in hue from ruby-red to
rose, the latter being the most common. No large red diamonds are
known, the largest being one of 32 carats in Vienna. Another famous one
is that in the Russian treasury, for which Paul I. paid one hundred
thousand roubles. It is of a ruby color. The finest green diamond known
is the "Dresden Green" preserved in the Green Vaults of Saxony. It was
purchased by August the Strong in 1743 for sixty thousand dollars. It
is apple-green in color and weighs 40 carats. Diamonds of yellow color
are comparatively common, many of the Cape diamonds being lowered in
value by possessing a yellow tinge. It is said that this injurious
yellow tinge can be overcome by dipping the stone several times in a
solution of potassium permanganate, the violet color of the latter
neutralizing the yellow of the diamond. The yellow tinge usually also
disappears in artificial light. Of large diamonds possessing a yellow
color the Florentine and the Tiffany are the best known. The color of
colored diamonds is generally permanent, but that of some is said to
fade on exposure to light. It can also be destroyed or changed by heat.
The luster of the diamond is a peculiar one, and such as is possessed
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