128 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



3-521 ; for colourless Cape diamonds 3'520, and for yellow diamonds from the same locality 

 3-524i. 



Determinations which give results much above or much below the mean value of the 

 specific gravity of diamond, namely, 3"52, and specially those which approximate to the 

 extreme values, 3'3 and 3'7, must be regarded either as inaccurate or as having been made 

 on impure material. 



Black carbonado has a much lower specific gravity than pure crystals of diamond, 

 values ranging from 3'14<1 to 3"416 having been determined. This is due to the porous 

 nature of the material, the numerous air spaces enclosed in its substance causing it to be 

 lighter, bulk for bulk, than are crystals of diamond. 



4. CLEAVAGE OF DIAMOND. 



When a diamond crystal is broken on an anvil by a blow from a hammer, or when it is 

 subjected to sudden changes of temperature, it breaks into a number of fragments which 

 are usually bounded by perfectly plane and bright surfaces. These surfaces of separation, 

 or cleavage, will be found to have always a definite direction in the crystal, being parallel to 

 one or more faces of the octahedron. If a chisel be driven into an octahedron of diamond 

 in a direction parallel to an octahedral face the crystal will be divided into two portions, 

 and the smooth, bright surfaces of separation will be parallel to the same octahedral face. 

 By suitably varying the position of the chisel, the crystal may be divided in the same way 

 into two other portions of which the surfaces of separation will be parallel to any other of 

 the faces of the octahedron. It is not possible, however, to produce a cleavage in a cube of 

 diamond which shall be parallel to the faces of the cube ; in these directions there will be 

 irregular fractured surfaces only. If the chisel is so placed that a corner of the cube is 

 removed, a cleavage surface will be produced, this being as before parallel to a face of the 

 octahedron, the faces of which also truncate the corners of the cube in the natural crystals. 

 The cleavage surfaces of all crystals of diamond, whatever be their outward form, are always 

 parallel to the faces of the octahedron, and in no other direction can plane cleavage surfaces 

 be obtained. Diamond thus possesses an octahedral cleavage only, which is perfect and 

 obtained with the greatest ease ; this gem may indeed be regarded as one of the most 

 perfectly cleavable of minerals. 



This perfect cleavage with plane even surfaces throughout can, however, be obtained 

 only in simple crystals. When we are dealing with an intergrowth of two or more crystals 

 twinned according to the spinel-law (Fig. 31, g to i) or irregularly grouped together, the 

 cleavage surfaces will have diflFerent directions in each individual, and will not pass uninter- 

 ruptedly from side to side of the stone as in simple crystals ; it is thus impossible to divide 

 such a stone by a single cleavage surface. 



From every simple diamond crystal, no matter what may be its external crystalline 

 form, there may be obtained by cleaving parallel to all the octahedral faces a cleavage 

 fragment having the form of an octahedron (Fig. 32). The great importance of this 

 property of cleavage in connection with the faceting of diamonds has already been touched 

 upon, and will be again refen-ed to under the special description of the process of diamond 

 cutting. The property of perfect cleavage is not, however, a desirable one from every point 

 of view, for it is often responsible for the appearance of incipient cracks in the stone, which, 

 if they further develop, seriously diminish its value. 



