26 



GENERAL CHARACTERS OF PRECIOUS STONES 



Precious Stones arranged according to Specific Gravity. 



B. CLEAVAGE. 



It has previously been briefly pointed out that crystal Hsed bodies are distinguished 

 rom amorphous bodies by certain peculiarities of internal structure. Whereas the substance 

 of an amorphous body possesses identical properties in every direction through it, the 

 substance of a crystallised body possesses different properties in diflferent directions. An 

 important character of crystallised bodies, and one which shows considerable variation in 

 different directions, is the degree of cohesion existing between the ultimate crystalline 

 particles of which the mass is built up. In many such bodies the cohesion in certain 

 directions is so feeble that a slight blow is sufficient to cause them to break into fragments. 

 On examination, these fragments will be found to possess perfectly plane surfaces, which are 

 the surfaces of minimum cohesion in the substance. This phenomenon is shown to per- 

 fection by calcite, a crystal of which, if allowed to fall, will break into fragments bounded by 

 perfectly bright and even surfaces. 



The best way to produce this separation or splitting along plane surfaces is to place a 

 chisel or knife edge on the crystal in the proper position, and to drive it in with a single 

 sharp blow from a hammer. 



