68 TERMINOLOGY. 



. 58. STRUCTURE. 



The internal structure of crystals depends upon the con- 

 nexion existing among their particles. It may be observed 

 by mechanically overcoming this connexion. 



In effecting this separation, smooth and polished surfaces are of- 

 ten produced in certain directions, and when a smooth surface is 

 thus produced in any direction, others may also be effected in the 

 same direction. Two or more of these being developed in a crys- 

 tal, give origin to plates or laminae, from whence the expression, 

 foliated structure or laminated structure, which has been applied 

 to such crystals.* In other directions, the application of force only 

 produces uneven faces, between any two of which thus produced, 

 no parallelism exists. 



The production of even and polished surfaces, in causing the sep- 

 aration of the pai ticles of crystals, is denominated their cleavage; 

 of uneven and irregular surfaces, \\\e\v fracture. The fracture of 

 crystals will be treated of hereafter. 



It is sometimes difficult to ascertain the cleavage of crystals, and 

 very often equally so, to discover their fracture. The particles of 

 Galena and Calcareous spar, separate so readily in the direction of 

 their cleavages, that it is almost impossible to effect a separation in 

 any other direction. On the contrary, others, as Quartz and Tour- 

 maline, separate in other directions with greater facility than in 

 those of their cleavage, so that it becomes difficult to observe even 

 the traces of its existence. 



<. 59. CLEAVAGE. 



A crystal is said to be cleavable, or to admit of cleavage, 

 if, by the application of mechanical force, it splits into par- 

 allel layers. 



* Lapidaries have long been acquainted with this property in many of 

 the gems, especially in the Diamond, which possesses a laminated struc- 

 ture in four directions; a peculiarity of considerable importance in the 

 cutting of this gem, since by means of it, flaws, and badly colored por- 

 tions, are easily got rid of, without the trouble of being ground down, 

 as in other gems. 



