Crystallisation
The diamond belongs to the isometric system of crystallography; the prevailing form is octahedral. It frequently occurs with curved faces and edges. Twin crystals (macles) are not uncommon. Diamond crystals are generally perfect on all sides. They seldom show irregular sides or faces by which they were attached to a support, as do artificial crystals of chemical salts; another proof that the diamond must have crystallised from a dense liquid.
The accompanying illustration ([Fig. 14]) shows some of the various crystalline forms of native diamonds.
FIG. 14. CRYSTALLINE FORMS OF NATIVE DIAMONDS.
To face p. 86.
No. 1. Diamond in the form of a hexakis-octahedron (the forty-eight scalenohedron), or a solid figure contained by forty-eight scalene triangles. According to Professor Maskelyne, this occurs as a self-existent form only in the diamond.
No. 2. Diamond in the form of a hexakis-octahedron and octahedron. From Sudafrika.
No. 3. Diamond in the form of octahedron with intersections.
No. 4. Diamond from Brazil.
No. 5. Diamond from Kimberley.
No. 6. Diamond from Brazil.
No, 7. A macle or twin crystal, showing its formation from an octahedron with curved edges.
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.
FIG. 15. TRIANGULAR MARKINGS ON NATURAL FACE OF A DIAMOND CRYSTAL.
FIG. 16. TRIANGULAR MARKINGS ARTIFICIALLY PRODUCED ON A DIAMOND CRYSTAL.
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[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 |