116 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



the action of these reagents in the way diamond does ; it will remain in them unaltered even 

 at high temperatures. 



On examining a partially burnt crystal of diamond, it will be seen that its edges and 

 corners are more or less rounded, and that its faces ai'e no longer brilliant but dull, rough, 

 and scarred. On crystals which are bounded by faces of the octahedron, special markings 

 are seen when such faces are examined with a lens or, better still, with a microscope. These 

 markings are regular triangular depressions like inverted pyramids, the bases being equilateral 

 triangles, of which the edges are in all cases parallel to the octahedral edges of the crystal, 

 as is shown in Fig. 31 r. The direction of these triangular pits is the reverse of that of the 

 natural depressions of diamond crystals, shown in Fig. 31 q, n, and o. Such pits may occur 

 singly on the face of the crystal, or they may be close together and in large numbers ; they 

 are of precisely the same chai-acter as etched or corrosion figures, such as may be produced 

 on the faces of other crystals by the action of fused alkalis, or of solvents such as water and 

 acid. These depressions may indeed in the present case be regarded as corrosion figures, 

 since they are produced by heating the diamond crystal in air or with saltpetre, the etching 

 agent being then hot oxygen gas or fused saltpetre. The production of etched figures is due 

 to the unequal action of the oxygen over the surfaces of the crystal, the crystal being 

 attacked first at isolated points on the faces, where the material is slowly consumed. 



As has been previously mentioned, a diamond heated away from contact with air 

 undergoes no diminution in weight. The experiment may be performed by packing the 

 diamond in charcoal powder in a closed crucible and heating it in an electric furnace. The 

 temperature may be as high as the furnace can produce, and may be maintained for any 

 length of time, yet the diamond will still remain unaltered in weight, since in the absence of 

 oxygen there can be no combustion of its substance. A prolonged exposure to great heat 

 does, however, produce other changes ; the surface of the diamond becomes blackened 

 and soft enough to leave a mark behind it when rubbed on paper. This is due to the 

 transformation to graphite of the surface material ; this change in state of the 

 substance of diamond to the other crystallised modification of carbon, namely graphite, 

 is however, brought about only when a very high temperature is reached. According to 

 G. Rose, who specially investigated the point, a diamond heated out of contact with air 

 undergoes no change whatever when heated to the temperature at which cast-iron melts, nor 

 even when exposed to the fiercest heat of a porcelain furnace. At higher temperatures, 

 about that at which bar-steel melts, or in the electric furnace, a superficial blackening and 

 conversion into graphite begins ; if the exposure to the temperature is prolonged sufficiently, 

 this conversion proceeds until the whole of the substance of the diamond is changed into 

 graphite, the original external form of the crystal being, however, still retained. 



The behaviour of diamond with respect to its alteration into graphite when heated 

 under other conditions, namely, in the presence of air, has not yet been thoroughly 

 investigated. While in some experiments no blackening, even at the highest temperatiu-es, 

 has been observed, in others this phenomenon has been seen ; in these cases, however, the 

 blackening may have been due to a sooty coating derived from the burning material which 

 supplies the source of heat and not to the alteration of diamond into graphite. Many 

 observers, including Lavoisier, have noticed that black spots are formed on the surface of a 

 diamond undergoing combustion. When combustion is again allowed to pi-oceed these 

 spots may disappear, or they may be still apparent at lower depths as the outer parts of the 

 stone slowly bm-n away. According to Rose, no alteration into graphite takes place when 

 a diamond is heated or burnt in a muffle or before the blowpipe, or perhaps even before the 

 oxyhydrogen flame. When placed in the focus of a concave mirror, however, or when 



