10 GENERAL CHARACTERS OF PRECIOUS STONES 



of these systems, to one or other of which every mineral and every crystallised precious stone 

 must of necessity belong. The names of the different crystal-systems with the number of 

 planes of symmetry characteristic of each are given below : 



1. The Cubic System with 9 planes of symmetry. 



2. The Hexagonal System „ 7 „ ,i 



3. The Tetragonal System „ 5 ,, ., 



4. The Rhombic System ,, 3 „ „ 



5. The Mouoclinic System „ 1 „ ., 



6. The Triclinic System „ ,, „ 



Sometimes the symmetry exhibited by a crystal is such that only half the typical 

 number of faces are developed. These derived forms are known as hemihedral, or half-faced 

 forms. These forms must be distingui'shed from those possessing the full number of faces, 

 which are known as holohedral, or full-faced forms. Again, from hemihedral forms may be 

 derived, by a development of only half the faces, another group, the members of which are 

 known as tetartohedral, or quarter-faced forms. The hemihedral and tetartohedral classes of 

 the different systems receive special names, which, however, need not be mentioned here. 

 All the holohedral and several of the hemihedral and tetartohedral classes are represented 

 among precious stones. 



All precious stones of the same kind, i.e., all diamonds, all emeralds, &c., exhibit forms 

 belonging to the same crystal system ; they all possess the same degree of symmetry, and all 

 show the same hemihedral or tetartohedral development if such is present. 



It not unfrequently happens that two similarly developed crystals of one and the same 

 mineral are so grown together as to be symmetrical with respect to each other about a 

 certain plane, one crystal being a reflection of the other in this plane, as, for example, is 

 shown for spinel in Fig. 60d. A regular grouping of two crystals in this way is known as 

 a twhi. Twins may generally be recognised by the presence of re-entrant angles between 

 the faces at the edge of the plane of junction of the two crystals. Simple crystal indi- 

 viduals do not show such re-entrant angles. Sometimes on the second crystal of a twin a 

 third individual may be grown in the sauje manner, thus giving rise to a triplet. Similarly 

 foxu- crystals grown together in a certain regular manner give rise to a quartet. Such 

 regular growths are often very complex, and it is then no easy matter to discover the mutual 

 relations of the several simple crystals. 



The principal crystalline forms are too important to be ignored ; they will be described 

 and figured below with the description of the various precious stones. To those who possess 

 even a small acquaintance with the laws and terms of crystallography, the description of the 

 different forms and their mutual relations will be easily intelligible ; to others, however, it 

 may present some difficulty. But as all bhe crystallographic details are collected together in 

 a small space, it is open to such persons to omit them. Though their conception of a 

 precious stone in its natural condition will, in such case, suffer, yet a fairly correct idea of the 

 aspect and crystalline form of uncut crystallised precious stones may be obtained from an 

 inspection of the figures. 



Amorphous substances, such as opal, which are incapable of assuming a crystalline 

 form, usually occur in irregular masses of indefinite shape, but rounded, spherical, botryoidal, 

 reniform, or nodular masses are also found. 



Crystallised bodies, and, consequently, many precious stones, are frequently not of 

 uniform structure throughout ; they do not consist of a single crystal individual, but of 

 several irregularly grown together. The compact mass which results from such a collection 



