28 GENERAL CHARACTERS OF PRECIOUS STONES 



cleavage also in one direction. Should one of the stones show a distinct cleavage, there can 

 then be no doubt that it is topaz and not aquamarine. If, however, no distinct cleavage 

 can be made out, the evidence must be considered as negative, since a cleavage face need not 

 necessarily be developed or outwardly visible even on a mineral which cleaves with great 

 facility. 



The cleavage of a mineral is not always expressed as a cleavage surface forming one of 

 the external boundaries to the stone. Fairly perfect cleavage is often indicated by the 

 presence of plane cracks running in a certain direction inside the crystal itself. Frequently 

 such crevices give rise to the brilliant rainbow colours of thin films ; the film here being air 

 or simply a vacuous space. The cleavage of the stone is thus manifested by these iridescent 

 colours in a very beautiful manner. On a surface parallel to which there is a very perfect 

 cleavage there is often to be seen a peculiar lustre resembling that of mother-of-pearl ; 

 this is limited to crystals possessing a perfect cleavage, and hence its occurrence may 

 be taken as an indication of the presence of such. Even in faceted stones cleavage 

 may be sometimes recognised by the iridescent colours and pearly lustre due to internal 

 plane fissures. 



From the aesthetic and commercial points of view, however, the presence of such cleavage 

 fractures in a cut stone is far from desirable, since they give rise to irregularities in the 

 reflection and refraction of light which seriously diminish the beauty and consequently the 

 value of the stone. The presence of cleavage cracks or " feathers " is a very bad fault in a 

 transparent precious stone, for a small and scarcely noticeable crack may in course of time 

 •extend and cause the stone to break into fragments. Rough stones showing any marked 

 cleavage cracks are useless for cutting, since in the process they will probably break. 



Stones which cleave with great facility should be treated with special care when 

 mounted as gems, for a fall or blow, or a sudden rise in temperature (arising perhaps from 

 immersion in hot water), may be sufficient to give rise to, or further develop, a cleavage crack, 

 which may result in the complete fragmentation of the stone. 



Although the property of cleavage in some cases leads to undesirable results, yet 

 considerable advantage may be derived from it in others. A stone with a distinct 

 cleavage, such as topaz, too large for a single gem, may be easily reduced by cleaving to any 

 desired size with no loss of material ; a stone not possessing this property must be sliced to 

 the required size, a process involving the expenditure of much time and trouble. Again, by 

 cleavage, portions of rough stones may be easily and quickly removed which otherwise would 

 have to be got rid of by grinding, a laborious and costly operation. Moreover, the cleavage 

 fragments can be utilised in the fashioning of smaller gems, and waste of material thus 

 avoided. 



The facile cleavage of the diamond is utilised very largely in the production of cut 

 stones. As we have already seen, the cleavage form of the diamond is an octahedron, and 

 this approximates to the shape of a brilliant, which is the form of cutting usually adopted 

 for the diamond. The first stage in the transformation of a rough diamond into the cut 

 stone is therefore the development of the octahedral cleavage form, and this is quickly and 

 easily performed owing to the ready cleavage of the mineral. This property of the 

 diamond then obviates both the necessity for the laborious and expensive process of grinding 

 in the production of the cut stone, and also the waste of material consequent on this 

 process. 



Fracture. — The fractured surface of a mineral not possessing the property of cleavage 

 is not plane, but uneven and irregular. The particular character of the fractured 

 surface, or fracture as it is called for brevity, is different in different minerals, and. 



