are
usually brighter. A ruby-red Diamond of ten carats is said to be
amongst the Russian crown jewels, and a rose-red one of three times
this size belongs to Austria. Blue is the rarest shade of all. It may
occur as a light or dark blue. Probably the most noted stone of this
colour is the " Hope " Diamond, of forty-four carats.
Brazil seems to have produced most of the coloured stones of note, though recently a fine red specimen was obtained in Borneo.
Many
Diamonds, when found, are coated with a coloured film, frequently of a
greenish tinge. This is particularly the case with Brazilian stones,
and as a rule is found only to be superficial, so that by cutting a
perfectly colourless stone may be produced. Some of the South African
stones are " smoky," especially on the dihedral and solid angles.
The
lustre of well crystallised Diamonds is adamantine in a splendant
degree. From this high lustre all grades may be found to greasy and
dull. Water-worn stones often show almost a metallic lustre. Crystals
are usually transparent, while Bort is only translucent, and Carbonado
is opaque. The degree of transparency varies greatly with the condition
of the surface. Thus water-worn crystals may be likened to ground
glass—seemingly translucent only; but if their surfaces be polished, as
in the process of cutting, the stone may be found to be transparent in
a high degree. Stones which show more than a certain degree of
cloudiness cannot be utilised as gems.
The refractive index is high, and dispersion is very high. Des Cloizeaux gives the following value of n—
Red. Yellow. Green.
2-4135 2-4195 2-4278
