Some crystals of diamonds have their surfaces beautifully marked with equilateral triangles, interlaced and of varying sizes (Fig. 15). Under the microscope these markings appear as hollow depressions sharply cut out of the surrounding surface, and these depressions were supposed by Gustav Rose to indicate the probability that the diamonds had at some previous time been exposed to incipient combustion. Rose pointed out that similar triangular striations appeared on the surfaces of diamonds burnt before the blowpipe. This experiment I have repeated on a clear diamond, and I have satisfied myself that during combustion before the blowpipe, in the field of a microscope, the surface is etched with triangular markings different in character from those naturally on crystals (Fig. 16). The artificial striæ are very irregular, much smaller, and massed closer together, looking as if the diamond during combustion flaked away in triangular chips, while the markings natural to crystals appear as if produced by the crystallising force as they were being built up. Many crystals of chemical compounds appear striated from both these causes. Geometrical markings can be produced by eroding the surface of a crystal of alum with water, and they also occur naturally during crystallisation. [Illustration: FIG. 15. TRIANGULAR MARKINGS ON NATURAL FACE OF A DIAMOND CRYSTAL.] [Illustration: FIG. 16. TRIANGULAR MARKINGS ARTIFICIALLY PRODUCED ON A DIAMOND CRYSTAL. To face page 88.] 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