DIAMOND: CRYSTALLINE FORM 119 



material. There is no single case in which the vegetable nature of a diamond enclosure has 

 been conclusively proved, although the attention of botanists has been more than once 

 directed to this point. 



All the minerals which have yet been mentioned as occurring as enclosures in diamond, 

 must have been formed before the diamond commenced its gradual growth around them. 

 There are other foreign bodies, however, which must have been introduced after the 

 formation of the diamond ; thus, water containing iron in solution has sometimes 

 penetrated the cracks and fissures of a diamond, and has left a brown deposit of limonite 

 filling up the crack or fissure. 



The enclosures of diamond do not invariably consist of solid matter. Not infrequently 

 there exist cavities in the substance of the diamond which may be vacuous or may contain 

 liquid ; these fluid enclosures are, however, usually of microscopic size. The liquid 

 they often contain does not, as a rule, completely fill the cavity, part of the space being 

 occupied by a bubble of gas, which is sometimes fixed in position and at other times 

 movable, thus clearly indicating the fluid nature of the contents of the cavity. In some 

 cases it can be safely infen-ed from the behaviour of the liquid when the diamond is heated 

 that it is liquid carbon dioxide ; this point will receive further consideration, however, 

 when we come to consider the origin of diamond. In other cases the properties of the 

 liquid point to its being water or a saline solution. 



Other cavities in diamond are quite empty or only filled with gas ; like the fluid 

 enclosures these are by no means rare, and, when present at all, occur in large numbers. 

 When observed under the microscope they appear quite black, especially at the visible 

 margins ; this is owing to the fact that the rays of light travelling through the stone are 

 almost totally reflected at the surface of separation of the substance of the diamond and the 

 bubble of gas, they therefore fail to reach the eye and the cavity appears dark. This is a 

 fruitful source of error, for such appearances are liable to be mistaken for solid enclosures of 

 a black colour. Such mistakes may be avoided, however, by careful observation, for the 

 outline of cavities is usually rounded, while that of solid enclosures is irregular an d 

 angular ; moreover, most cavities allow the passage of some light, at least in the centre ; 

 they will therefore appear to have a bright centre surrounded by a dark border, which 

 would not be the case with solid enclosures. The presence of these cavities is of practical 

 significance, since to them is due the cloudiness of the diamond, and those faults which have 

 been already considered under the name "silk." From a theoretical point of view, they will 

 no doubt help to throw light on the obscure question of the origin of diamond. 



2. CRYSTALLINE FOEM OP DIAMOND. 



The diamond is one of the most perfectly crystallised of minerals. Almost every 

 single stone is bounded by more or less regularly developed faces. Massive specimens 

 without crystal-faces are scarcely ever found, and when such are met with they are, as a rule, 

 fragments of large crystals or rounded pebbles, of which the original external crystalline 

 form has been destroyed. As is usually the case with embedded crystals, that is, those 

 which have grown embedded in the mother-rock, most diamonds are bounded on all sides by 

 crystal-faces. Sometimes, however, irregular areas, by which the crystal might have been 

 attached, can be made out with more or less certainty. 



The faces of diamond crystals differ from those of most other crystallised minerals, in 

 that they are, as a rale, much curved and rounded instead of being perfectly plane, as is 

 usually the case. This curvature is due to the mode of growth of the crystal, and not to 

 subsequent attrition, as might be thought. It renders the exact determination of a crystal. 



