THE GARNET GROUP 347 



Fig. 29i, are usually delicately striated in the direction of their length. Sometimes the 

 faces of the rhombic dodecahedron are more largely developed than are the truncating faces 

 (Fig. 296) ; at other times the i-everse is the case, and the rhomb-shaped faces are small. 

 These truncating faces belong to the icositetrahedron (Fig. 69c), a simple form, uncombined 

 with others, frequently taken by garnet. The edges of the rhombic dodecahedron are in 

 many cases not- only truncated by the icositetrahedron, as in Fig. 69b, but the edges of 



i. c. 



Fig. 69. Crystalline forma of giu-net. 



intersection of these two forms are further truncated by delicately stiiated faces, the result 

 being a form like Fig. Gfdd. This second series of faces are those of a hexakis-octahedron, 

 a solid bounded by forty-eight faces, which is the greatest number possible on any single 

 uncombined form. The simple hexakis-octahedron uncombined with other forms has not 

 been observed in garnet. The mineral rarely takes a form other than those mentioned ; 

 those forms having a lesser number of faces, such as the octahedron and the cube, which, 

 as a rule, are commonest in other minerals, are rarely seen in garnet. 



The cleavage of garnet is more imperfect than in most other minerals ; the fracture is 

 sub-conchoidal to uneven. It is fairly hard, but this character varies in different varieties. 

 All red garnets, which are the varieties chiefly used as gems, are harder than quartz but 

 less hard than topaz, that is to say, for red garnets H = 7 — 8. The hardness of some green 

 garnet is rather less ; the demantoid, for example, which is sometimes used as a gem, has a 

 hardness of Q^ only, and is scratched by quartz ; it is sufficiently hard, however, to scratch 

 glass, so that it may be distinguished by this means from glass imitations. The pure 

 emerald-green calcium-chromium garnet known to mineralogists as uvarovite is rarely cut as 

 a gem, but is very nearly as hard as topaz. Garnet in the form of powder is a valuable 

 grinding agent for precious stones and other hard substances, and is also used in the 

 manufacture of the so-called emery-paper. It is the most widely distributed of any mineral 

 with a hardness greater than that of quartz, and can be sold in large quantity at a low 

 price. Good stones free from fissures serve for the construction of the pivot supports of 

 \vatches, &c. 



The specific gravity of garnet is another feature which varies to a lai'ge extent in 

 different varieties. The variation is due to the diversity in chemical composition, the 

 greater the proportion of heavy metal, such as iron, present in any one variety, the heavier 

 ^vill be that variety. The calcium-aluminium garnet, with a specific gravity of 3"4, is the 

 lightest, while iron-aluminium garnet, with a specific gravity of 4!"3, is the heaviest. The 

 specific gravities of other varieties lie between these two extremes, and will be mentioned 

 with the description of each. The specific gravity is a feature which enables us to 

 distinguish garnet easily from other stones of similar appearance and from glass imitations. 



The colour of garnet is not due to any intermixed pigment, but depends on the 

 chemical composition of the mineral itself This being the case, the colour, whatever it 



