CLEAVAGE 



S7 



Fig. 8. Cleavage of calcite. 



The direction within a crystal, along which there is minimum cohesion, is known as the 

 direction of cleavage, and the plane surface of separation is known as the cleavage face. The 

 cleavage directions and faces are always identical both in number and direction in all 

 specimens of the same mineral species. The ease with which cleavage takes place, and the 

 perfection of the resulting cleavage faces, are also constant in all 

 specimens of the same species, but different in different species. 

 Some minerals exhibit little or no cleavage, while in others, notably 

 calcite, cleavage is produced very easily, and the surfaces of separa- 

 tion are perfectly bright, smooth, and even. 



Among precious stones possessing the property of cleavage to 

 a high degree may be mentioned topaz, which cleaves in one 

 direction, and diamond which cleaves in four directions. In others, 

 as, for example, emerald, cleavage takes place with difficulty, and 

 the cleavage faces are uneven and frequently interrupted by 

 irregular areas. Quartz, garnet, tourmaline, &c., are other 

 examples of pi-ecious stones possessing no distinct cleavage ; the 

 difference of cohesion in different directions being so small that the stones will not split 

 along plane surfaces. 



In amorphous bodies, as, for instance, opal, the degree of cohesion between the constituent 

 particles, like all other physical characters, is identical in every direction, so that here plane 

 cleavage faces are impossible, and as a fact never occur. When a body shows sure indications 

 that it possesses the property of cleavage, we are safe in inferring from that fact alone that 

 the material of which the body is composed is crystallised and not amorphous. Hence it is 

 sometimes possible to distinguish between a genuine crystallised precious stone and a glass 

 imitation, since glass, being amorphous, can have no cleavage. 



If in the same crystal there are three or more directions of cleavage, it will then be 

 possible to develop out of it by cleavage a body bounded entirely by cleavage faces ; such a 

 body is known as the cleavage J'orvi of that particular crystal. Calcite, for example, cleaves 

 with equal facility in three directions, inclined to one another at equal oblique angles. It 

 is therefore possible to obtain from any crystal of calcite a cleavage fragment having the 

 form of a rhombohedron, a solid figure which resembles a cube with two of the opposite 

 corners pressed together. In the same way, the four cleavages of diamond will give a cleavage 

 form identical with the regular octahedron. 



Such cleavage forms resemble natural crystals in possessing plane regular faces, but 

 whereas in crystals these faces are the result of natural growth, in cleavage forms they have 

 been produced by artificial means. In connection with the cutting of precious stones and 

 the purposes to which they are to be applied, a knowledge of the cleavage possessed by 

 different stones is most desirable. The property of cleavage considered from this point of 

 view will be treated in detail under the descriptions of individual stones. 



Cleavage frequently affords a simple means by which a stone in the rough condition 

 may be identified or distinguished from others of similar appearance. As we have already 

 seen, the cleavage directions and faces are always identical in number, direction, and quality 

 in all specimens of the same mineral species, and in general differ from those of other species. 

 The cleavage of a stone is thus one of its characteristic and distinguishing features. As an 

 example of the use which can be made of this character, let us suppose a case in which it 

 might be very difficult to decide which of two stones is an aquamarine, and which a certain 

 colour-variety of topaz, both being of a sea-green colour and very similar in general appear- 

 ance. Aquamarine has a very imperfect cleavage in one direction, while topaz has a perfect 



