472 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



growtli it may happen that the faces of the pyramid all differ in size and are arranged in no 

 particular order as to size. The small faces represented in Figs. 85b and c, between the 

 prism- and the pyramid-faces are very often absent ; when present they occur in regularly 

 developed crystals on the alternate upper and lower corners (see Figs. 856 and c), but this 

 arrangement is often disturbed by the twinning of the crystals (P^ig. 85d), 



Quartz crystals occur either embedded in the mother-rock, when they are developed on 

 all sides, as in Fig. 85a, or attached to a rock surface, in which case only the free end is 

 developed (Figs. 856 — d). The attached crystals sometimes form fine druses, as represented 

 in Plate XVII. In this illustration the crystals are of an elongated prismatic habit, but 

 sometimes, when only the terminal pyramid-faces are developed, the surface of the rock 

 appears covered with close-set crystals. 



Quartz occurs very commonly also in compact masses. As isolated irregular gi"ains it 

 forms an important constituent of many widely distributed rocks, such as granite, gneiss, &c., 

 and, in more rounded grains, of sand, sandstone, grit, &c. Frequently, also, aggregations of 

 microscopically small quartz grains are met with ; these aggregations constitute the different 

 varieties of compact quartz, such as hornstone (which includes the green chrysoprase), jasper, 

 and others. Columnar aggregates are not uncommon, and when the columns or fibres are 

 very slender the mass has a fibrous structure, as, for example, in tiger-eye. Altogether these 

 compact varieties of quartz furnish quite a large number of ornamental stones, each of which 

 will be described in its proper place. 



The mineral has no distinct cleavage. The fracture of crystals and crystalline masses 

 is conchoidal, almost as perfectly conchoidal as that of glass ; in compact aggregates it is 

 uneven to even, sometimes splintery. Quartz stands seventh on Mohs' scale of hardness ; 

 though not as hard as precious stones properly so-called, it is harder than any other widely 

 distributed mineral, and scratches the majority with ease, as it does also window-glass. 

 Steel or iron when struck against quartz gives rise to bright sparks, hence the German 

 name " Feuerstein " (fire-stone) for flint, a compact variety of this mineral. On the other 

 hand, quartz is scratched by most of the more valuable precious stones, being surpassed in 

 hardness by topaz, corundum, and diamond, &c. The mineral is very brittle, and splinters 

 may be broken off, at any rate, from large crystals, with little difficulty ; some of the fine 

 grained and compact varieties, however, are much less easily broken. 



The specific gravity of pure quartz is 2'65, so that in this respect also quartz differs from 

 opal, which is appreciably lighter. It should be observed that the value given above applies 

 only to pure material, and that the specific gravity of impure varieties may be rather less 

 or rather greater. 



Quartz is infusible before the ordinary blowpipe, but can be fused in the oxyhydrogen 

 flame. It is completely dissolved by hydrofluoric acid, but is unattacked by other acids ; and 

 is scarcely affected at all by a solution of caustic potash, in which, however, opal readily 

 dissolves. On being rubbed it becomes electrified and retains its charge for nearly an 

 hour. 



The many varieties of quartz which occur in nature differ widely in external appear- 

 ance, these differences depending on variations in structure, lustre, transparency, or colour. 



Generally speaking, the lustre of quartz is of the vitreous type; some specimens 

 exhibit a greasy lustre (greasy quartz), while finely fibrous aggregates sometimes possess a 

 very good silky lustre. All varieties of quartz are susceptible of a high polish ; the lustre 

 of natural faces and fractured surfaces is often not particularly strong, but is much 

 heightened by cutting and polishing. 



In its purest condition quartz is perfectly transparent and colourless, and in many 



