CRYSTALLINE FORM 9 



development in all directions is impossible. Or, again, as often happens, in extricating a 

 crystal from the matrix, a blow from the hammer may destroy some (if the plane faces of the 

 specimen. Moreover, in the process of cutting and polishing precious stones, the natural 

 plane faces are always destroyed. In all these cases, however, the substance still possesses 

 the internal structure characteristic of a crystallised body. The essential difference between 

 a crystallised and an amorphous body lies in their internal structure, on which depends the 

 character of the substance. The presence of plane faces in the crystallised substance is 

 merely the outward expression of its internal structure. 



A crystallised body which shows no regular boundaries is said to be crystalline or 

 massive. When these boundaries are present the body is termed a crystal. Portions of 

 crystalline, massive material cannot be distinguished in their external form from an 

 amorphous substance, but their internal structure shows a very essential difference, which 

 will be described later. A crystal, however, on account of its regular boundaries can never 

 be confused with an amorphous body. 



The knowledge of crystals and the laws governing the relations between their faces 

 belongs to the special science of cry.stallography. A knowledge of the subject is essential to 

 the correct understanding of the natural relations of minerals, including also precious 

 stones. 



It has been esrablished that pach crystallised substance, including precious stones, 

 having a definite chemical composition, has also a crystalline form which is characteristic of 

 the substauce, or to be more correct, it may exhibit a series of crystallised forms related in 

 such a way that each may be derived from another. Moreover, bodies of different chemical 

 composition will in general be characterised by different crystalline forms, having, as a rule, 

 no mutual relations. 



Hence it is possible to distinguish bodies not only by their chemical composition but 

 also bj their crystalline form, and this applies equally to precious stones. It is thus obvious 

 that a knowledge of the crystallographic relations of precious stones is not only of 

 theoretical importance, but also of the highest practical importance, for it would enable a 

 buyer of rough stones to distinguish a genuine from a false stone by the form alone, thus 

 avoiding injury to the stone. This method of identification, however, is applicable only when 

 the specimen is crystallised. In the case of massive or crystalline material, the data for its 

 scientific determination can only be obtained from the physical characters of the specimen. 



The science of crystallography is not one of which the general principles can be 

 conveyed in a few words. Generally speaking, a complete and thorough study of the subject 

 is necessary to obtain a knowledge of practical value. Since a detailed account of the 

 science of crystallography is quite outside the scope of the present work, the reader must be 

 referred to special works on the subject and to the various text-books of mineralogy, which 

 usually contain a section devoted to crystallography. It will, therefore, be assumed in what 

 follows that the reader possesses a knowledge of at least the elements of the subject, and is 

 further acquainted with the elements of those sciences, such as chemistry, physics, geology, 

 the aid of which is necessary in the study of minerals and precious stones. 



It may be stated briefly here that all crystals with few exceptions can be cut by a plane 

 into two equal parts, having the same relation between them as exists between an object 

 and its imawe in a mirror. Such a plane is known as a plane of symmetry, and crystals of 

 different substances possess different numbers of these planes. The greater the number of 

 planes of symmetry possessed by a crystal the higher its degree of symmetry. Those crystal 

 forms which may be cut in the same manner by the same number of planes possess the same 

 degree of symmetry, and are grouped together into the same crystal-system. There are six 



