THEIR HISTORY. 99 



puzzle the mineralogist with a multitude of beautiful geometric 

 shapes, among which it is indeed difficult to know where to begin. 

 We have a crystal of eight triangular sides, with sharp angles and 

 straight edges, in form like two square pyramids, joined base to 

 base ; this is the regular octahedron ; we have one of 6 sides, a 

 perfect cube; another with curved edges and 24 sides; another with 

 1 2 sides ; another of the shape of a cocked hat, having an irre- 

 gular edge, this is a double crystal or twin. Some have their sides 

 beautifully smooth and polished ; others striated, or covered with 

 little triangular pits, and presenting a grey, semi-opaque appear- 

 ance ; some are coated with a dense colouring matter that appears 

 to have entered into the outer layer of the crystal, and effectually 

 prevents the interior being seen. Very frequently, a stone so 

 coated, and appearing, when found, deep green and semi-trans- 

 parent, on being cut produces a fine brilliant of the purest white. 

 These regular forms are, however, comparatively rare, as by far the 

 greater part of the Diamonds found are broken chips and frag- 

 ments of every possible shape. All this multitude of different 

 shapes are, however, as stated above, derived or built up from 

 one normal form, the regular octahedron. The question will 

 be asked, " Why should the octahedron be considered the 

 normal form any more than another — say the cube ? " The 

 answer is, that the cube can be cleaved into the octahedron, but 

 the octahedron cannot be cleaved into the cubic form. For this 

 reason, that whatever is the shape of the Diamond, the cleavage 

 planes are always the same — viz., in four directions parallel to the 

 four pairs of faces of the octahedron, and in no other direction is 

 it possible to split a Diamond. It consequently follows that in 

 cleaving (see Plate) one can only produce faces of an octahedron. 

 The process of cleaving might fairly be called unmaking a Dia- 

 mond, for as Nature has built up the crystal layer upon layer, 

 so the cleaver takes off layer after layer till he lays bare the 

 original shape. 



The Diamond, being of the tesseral system of crystallisation, 

 has consequently three equal axes intersecting each other at right 

 angles, and around these three axes the ultimate carbon atoms 

 that are to form the crystal, group themselves according to an 

 invariable law, in triangular laminae, resting at definite angles with 



