[Illustration: FIG. 15. TRIANGULAR MARKINGS ON NATURAL FACE OF A DIAMOND CRYSTAL.]
[Illustration: FIG. 16. TRIANGULAR MARKINGS ARTIFICIALLY PRODUCED ON A DIAMOND CRYSTAL.
PHYSICAL AND CHEMICAL PROPERTIES OF THE DIAMOND
I need scarcely say the diamond is almost pure carbon, and it is the hardest substance in nature.
When heated in air or oxygen to a temperature varying from 760° to 875° C., according to its hardness, the diamond burns with production of carbonic acid. It leaves an extremely light ash, sometimes retaining the shape of the crystal, consisting of iron, lime, magnesia, silica, and titanium. In boart and carbonado the amount of ash sometimes rises to 4 per cent, but in clear crystallised diamonds it is seldom higher than 0·05 per cent. By far the largest constituent of the ash is iron.
The following table shows the temperatures of combustion in oxygen of different kinds of carbon:
°C. Condensed vapour of carbon 650 Carbon from sugar, heated in an electrical furnace 660 Artificial graphites, generally 660 Graphite from ordinary cast-iron 670 Carbon from blue ground, of an ochre colour 690 Carbon from blue ground, very hard and black 710 Diamond, soft Brazilian 760 Diamond, hard Kimberley 780 Boart from Brazil 790 Boart from Kimberley 790 Boart, very hard, almost impossible to cut 900
Diamonds vary considerably in hardness, and even different parts of the same crystal differ in their resistance to cutting and grinding.
Beautifully white diamonds have been found at Inverel, New South Wales, and from the rich yield of the mine and the white colour of the stones great things were expected. In the first parcel which came to England the stones were found to be so much harder than South African diamonds that it was at first feared they would be useless except for rock-boring purposes. The difficulty of cutting them disappeared with improved appliances, and they now are highly prized.
The famous Koh-i-noor, when being cut into its present form, showed a notable variation in hardness. In cutting one of the facets near a yellow flaw, the crystal became harder and harder the further it was cut, until, after working the mill for six hours at the usual speed of 2400 revolutions a minute, little impression was made. The speed was increased to more than 3000, when the work slowly proceeded. Other portions of the stone were found to be comparatively soft, and became harder as the outside was cut away.
