24 A Book of Precious Stones
ing
apparel. The diamond is also brittle so that it may be easily
fractured, especially at the girdle, by striking it a blow against some
hard substance, and in a steel mortar with a steel pestle it may be
reduced to powder. By what process in Nature's workshop carbon was
crystallised into the diamond is unknown, but scientific
investigators agree that the process was slow and a prime factor was a
titanic pressure.
The
specific gravity of the diamond is 3.52; hardness, 10; crystallisation,
isometric; cleavage, octahedral and perfect; refraction simple, with
an index of 2.439; a high dispersive power; lustre, brilliant
adamantine; is combustible though infusible; electric, positively, by
friction and a non-conductor of electricity; it is phosphorescent and
does not polarise light.
There
are three forms of diamonds: crystallised, used as gems;
crystalline—imperfect crystallisation,—harder than crystals, termed
bort (a word also applied to chips, waste, and stones unfit for
cutting); and carbonado, steel gray or black, shapeless, and without
cleavage.
To
the diamond's surpassing property of dispersing light, or dividing it
into coloured rays, is due that fascinating flash of prismatic hues
termed its fire. The stone's wondrous brilliancy
