DIAMOND: CRYSTALLINE FORM 



121 



peculiarities, however, render the hemihedrism of the crystals open to question, and some 

 authorities prefer to consider them as holohedral. All the typical simple forms of the cubic 

 system have been observed in diamond crystals, either alone or in combination with other 



Fig. 31, li-s. Crystalline forms of diamond. 



forms. Some of the more commonly occurring forms, which will be described in some 

 detail, are shown in Fig. 31, o-jo. 



Crystals having the form of a regular cube (Fig. 31 a) occur very frequently, but are 

 usually small ; this habit is specially characteristic of Brazilian crystals, and is rarely met 

 with in specimens from other localities, especially the Cape. The faces of the cube are 

 always dull and rough, and show a shallow depression, increasing in depth towards the 

 centre of the face. The roughness is due to the presence of square-based, pyramidal 

 depressions placed diagonally on the cube face ; these are usually small, but may be of 

 fair size. They occur more or less isolated or closely aggregated together (Fig. 31 a). 

 When observed with a lens or, better still, under the microscope, the pyramidal faces 

 bounding the shallow depressions may be distinctly seen ; they are marvellously plane and 

 smooth, but just as frequently rough and irregular, and between these two extremes all 



