100 DIAMONDS AND THEIR HISTORY. 



the axes, and forming planes that are the planes of the octahe- 

 dron ; and it is only by laminae laid on these octahedral faces that 

 increase takes place. It is, therefore, evident that if additions are 

 made to some faces of the original crystal and not to others, the 

 shape of the crystal will be altered ; also, if a series of laminae, 

 decreasing gradually in size, are piled up on the triangular faces of 

 an octahedron, each face will be trisected, and the octahedron 

 crystal changed into a crystal of 24 sides (see Plate). So it is 

 with al] other forms ; they are all built up by additions to some 

 or all of the original faces of the octahedron. 



I have spoken continually of cleaving, and should perhaps 

 have explained the process before. First, a cleavage plane is 

 found by examination of the surface-indications on the stone ; 

 then, having ascertained the directions in which it is possible to 

 cleave the stone, the workman fixes it by means of cement on a 

 handle. This he holds in his left hand, and with a sharp splinter 

 of Diamond, similarly fixed in a handle held in his right hand, he 

 abrades on the Diamond to be cleaved a small notch over the 

 plane through which he intends to split. This done, he places the 

 edge of a knife in the notch, and a gentle tap causes the diamond 

 to separate. Although it is so easily split in this manner, yet a 

 heavy blow from a hammer would probably fail to break it. 



Having considered the outward form of the Diamond, there 

 remains its physical qualities to be investigated. First in import- 

 ance is its hardness, which distinguishes it at once from any other 

 stone. It is the hardest substance in nature, and can only be 

 scratched by itself, while by it everything can easily be abraded. 

 It is this quality of supreme hardness that gives the Diamond its 

 value to the engineer for rock-boring, the glazier for glass-cutting, 

 the miller for dressing his mill-stones, and the lapidary for cutting 

 other stones. 



The specific gravity of the Diamond is 3-55, being character- 

 istically light (although so much harder, it is lighter than the 

 Sapphire). This, in conjunction with its great refractive power, as 

 before stated, first led to the theory that it was combustible. Of 

 its optical properties, I have not space in this paper to say more 

 than that it refracts light more than any other gem, which quality, 

 with its capacity, thanks to its hardness, of receiving the brightest 



