68 GENERAL CHARACTERS OF PRECIOUS STONES 



titaniferous variety, namely iserine, takes when polished a very brilliant lustre, and is 

 sometimes used for ornamental purposes. The magnetic character of magnetite distinguishes 

 it from other black stones, all of which are either non-magnetic or very feebly influenced 

 by a magnet. 



D. OCCURRENCE OF PRBCIOTJS STONES. 



A complete account of precious stones must include a consideration of the localities at 

 which they are found and the conditions under which they occur in nature. These subjects 

 will be dealt with in a general way here, and again more in detail with the special description 

 of each precious stone. 



Precious stones, like other minerals, have two distinct modes of occurrence. They may, 

 on the one hand, be found at that spot in the earth's crust where they had their genesis, or, 

 on the other hand, owing to the weathering and breaking down of the rocks and the action of 

 transporting agencies, we may find them in secondary deposits far from their original home. 



Precious stones, in their primary situation, frequently form a constituent of the rocks 

 which make up the earth's crust at that place. They are embedded in the so-called mother- 

 rock, and were formed at the same time as the other constituents of the rock. Under such 

 conditions, stones sometimes show regularly developed crystal-faces, but more frequently 

 their boundaries are irregular and distorted. A perfectly developed crystal of red garnet 

 (almandine) embedded in its mother-rock of gneiss is shown in Plate XIV., Fig. 3, while 

 Fig. 69 shows the completely developed crystal after being isolated from the mother-rock. 



Many precious stones and minerals, however, are found not completely embedded in the 

 rock-mass but attached to the walls of cavities in the rock and projecting freely into the 

 interior space. These cavities may be either completely enclosed by the rock, in which case 

 they are of various shapes and sizes, or they may partake more of the nature of cracks and 

 fissures penetrating the rock and varying in width and length between wide limits. The 

 formation of minerals found inside such cavities is always of later date than that of the rock- 

 mass itself. Such later-formed minerals may completely fill a cavity or fissure, or they may 

 form a more or less thick incrustation on its walls. 



Such cavities lined with crystals are known as drusy cavities or druses. Crystals 

 detached from a drusy cavity will show a broken surface at the end by which they were 

 attached to the wail of the cavity, but in other directions they will be perfectly developed in 

 accordance with the type of symmetry peculiar to them. These attached crystals differ in 

 this respect from the embedded crystals, mentioned above, which latter are equally developed 

 on all sides. 



The quartz crystals shown in Figs. 85 b — d, are examples of attached crystals broken 

 away from their underlying matrix, while Fig. 85 a is a representation of an embedded 

 quartz crystal, equally developed on all sides. In Figs. 85 b — d, the irregularly broken 

 point of attachmenb of each crystal is directed downwards and is fairly large; it is 

 sometimes, however, quite small and may be hardly observable. A group of crystals, 

 of the variety of quartz known as rock-crystal, such as frequently occurs in crevices and 

 fissures in the gneiss of the Alps, is shown in Plate XVII. 



More important than the occurrence of precious stones in primary rocks is their presence 

 in loose, secondary deposits, which have been derived from the weathering and breaking 

 down of primary rocks, and are known as gem-sauds or gem-gravels. 



The mother-rock, in which the precious stones were originally formed, has been exposed 

 to the action of abmospheric agencies, rain, frost, &c., and has become weathered at the 



