[CHAPTER VIII]
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 |
Hardness
Diamonds vary considerably in hardness, and even different parts of the same crystal differ in their resistance to cutting and grinding.