DIAMOND: HARDNESS 129 



5. HARDNESS OP DIAMOND. 



In respect to its hardness, diamond stands alone among all other substances, whether 

 natural or artificial. The hardness possessed by some artificial substances, such, for example, 

 as crystallised boron and carborundum, does, however, approach that of diamond. This 

 gem stands at the head of Mohs' scale, and receives the number 10 as a measure of its 

 hardness. Between this and the next hardest natural substance, namely corundum (ruby 

 and sapphire), there is a wide gap, the difference in hardness between diamond and 

 corundum being far greater than that which exists between corundum and talc, the softest 

 of all minerals. The unique degree of hardness thus possessed by diamond renders it easily 

 recognisable, since it scratches all other substances without exception, and is itself scratched 

 by none. 



It is a remarkable fact that degrees of hardness exist in diamonds among themselves, 

 this being shown by the fact that diamonds from one locality are capable of scratching 

 those from other localities. Thus the Australian stones are harder than those from South 

 Africa, which are said to be the softest of all diamonds ; and the beautiful black diamonds 

 of Borneo are harder than those of other colours. It is also remarkable that many South 

 African diamonds gradually assume their characteristic hardness only after a more or less 

 prolonged exposure to air. 



Diamond forms no exception to the general rule that the hardness of a crystal is not 

 everywhere the same. It has been observed that the powder obtained by rubbing the 

 surface of diamond crystals in the operation of bruting, which will be described in the 

 section on diamond cutting, is more efficacious in the process of grinding than is the powder 

 obtained bj- pounding up large fragments of diamond. It may naturally be inferred from 

 this, that diamond crystals must be harder on the exterior than in the interior. On the 

 surface itself, however, differences in hardness are distinctly perceptible, some faces of the 

 crystal being more easily scratched than others, while on each face there are certain 

 directions along which scratching may be more readily effected than in others. This being 

 so, it follows that the process of grinding will also be more difficult in certain directions and 

 on certain parts of the stone than in others. This subject will, however, be treated more 

 fully in the section devoted to diamond cutting. 



The great degree of hardness possessed by diamond renders it exceptionally suitable for 

 use in personal ornament, since the sharpness of the edges and corners of the cut stone and 

 the lustre of the polished facets are retained in spite of long continued wear. 



Several important technical applications of diamond depend on the enormous hardness 

 it possesses ; these will be fully discussed in a special section, and we need only mention 

 here the use of diamond powder in the cutting of the harder precious stones and of diamond 

 itself. The hard Australian diamonds, however, are unattacked by the powder of other 

 softer diamonds, they can only be worked by the help of their own powder. 



In spite of their enormous hardness, the diamond crystals found in river-sands and 

 gravels often show signs of wear and tear, their edges and corners being rounded and their 

 surfaces dull and roughened. This has been effected by long ages of grinding against the 

 pebbles and quartz-grains, and occasionally the precious stories of river-sands and gravels ; 

 thus even diamond itself cannot escape the action of time. 



The hardness of diamond has often been confused with its frangibility or brittleness. 

 It has been supposed, especially in ancient and mediaeval times, that hammer and anvil 

 may be shattered but not the diamond which lies between. This statement was made by 

 Pliny, the great naturalist of ancient days, who was killed in 79 a.d. at the first historic 



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