4>M 



SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



THE FELSPAR GROUP. 



The felspar group includes a number of minerals which are important constituents of 

 the earth's crust, through which they are widely distributed. Felspars in general are 

 cloudy, opaque, and dull in colour, so that they possess none of the characters which would 

 lead to their application as gems, or even as ornameutal stones. There are some few 

 exceptions to the rule, however, and of these a detailed account, preceded by a general 

 description of the characters of felspars, so far as they are of interest for our present purpose, 

 is given below. 



All the members of the group are silicates, that is to say, compounds of silicic acid 

 and bases, and in all of them alumina is present. Beside these constituents there is 

 present also either potash, soda, or lime, or soda and lime together. We may, therefore, 

 distinguish potash-felspar, soda-felspar, lime-felspar, and lime-soda-felspar; in the latter, 

 either the lime or the soda may predominate in different varieties. These felspars of 

 different chemical composition have received special names, some of which are mentioned 

 below. 



The felspars very frequently occur in well-developed crystals, often of considerable 

 size. Potash-felspar crystallises in the monoclinic, and the other kinds in the triclinic 

 system. All are very similar, however, in general form, and differ essentially only in the 



Fig. 80. Crystalline forms of felspar. 



size of the angles between the faces, and this difference at most amounts to but a few 

 degrees of arc. Some of the forms taken by felspar are represented in Figs. 80a to c. 

 In each case there is a rhombic prism, which in the simplest crystal (Fig. 80a) is terminated 

 by two obliquely inclined faces. Other faces are frequently developed, and there is nearly 

 always a pair of parallel faces truncating the acute prism edges (Fig. 806). Very 

 frequently two or more individuals grow together in accordance with various twin-laws, and 

 give rise to complex groups (Fig. 80c). 



Felspars frequently occur as constituents of rocks, such as granite, gneiss, trachyte, &c., 

 when they usually take the form of irregular grains, which are embedded in the rock-mass. 

 They also occur, however, as regularly developed crystals, which are attached to the walls 

 of crevices or cavities in rocks of the same character, often giving rise to druses of great 

 beauty. 



There are certain physical characters which all the felspars possess in common. All 

 have two good and easily developable cleavages ; one, which is very perfect, is parallel to 



