362 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



in situ in the underlying mother-rock. It is distinguished as Bobrovka garnet from the 

 loeahty at which it is found. 



The demantoid occurs here, with dolomite, a little clayey material, and magnetite, in 

 veins of chrysolite, which penetrate a peculiar grey to greenish-grey serpentine rock ; it is 

 found sometimes also in the serpentine itself. The garnet is embedded in this fibrous 

 chrysolite and coated with a layer of the same ; it occurs either as isolated irregular grains 

 or more commonly in nodules, the surface of which is irregularly grooved and furrowed 

 (Plate XIV., Fig. 9). These nodules measure from J to 2 inches across and are greasy and 

 cloudy in appearance. Each is built up of a large number of irregular grains of demantoid 

 packed closely together, but separated from each other by a coating of serpentine. Each 

 grain has a brilliant lustre and a perfectly conchoidal fracture. As a rule, each nodule is 

 divided by deep, prominent grooves into a small number of portions, and the grains which 

 build up these several portions are separated by finer grooves. Distinct crystals are rarely 

 found ; the rhombic dodecahedron and icositetrahedron, and also combinations of these two 

 forms, have nevertheless been observed. The rounded outline of the grains sometimes 

 appears to be due to the combination of numerous imperfectly developed crystal-faces. 



Demantoid is frequently cut and worn as a precious sbone at the place of its origin, 

 that is to say, in the Urals and elsewhere in Russia ; outside that country it is little used. 

 It is cut en cabochon (Plate XIV., Fig. 10), and frequently also in various faceted forms. 

 On account of its yellowish-green colour, the shade which is most frequently seen, demantoid 

 was at first thought to be chrysolite, and is even now known by this name in the Urals. It 

 may be distinguished from chrysolite, however, by its single refraction and high specific gravity, 

 demantoid sinking in the heaviest liquid (sp. gr. = 3"6) while chrysolite (olivine) floats. 



The chromiferous emerald-green variety of demantoid is very similar in appearance to 

 the emerald, and is therefore sometimes called " Uralian emerald"; this term, however, is 

 somewhat misleading, since true emeralds are also found in the Urals. The characters 

 mentioned above as serving to distinguish demantoid from chrysolite also serve to 

 distinguish it from emerald. The manner in which demantoid occurs precludes its extensive 

 use as a precious stone, the grains which build up the nodules described above being always 

 very small. Were it not for this fact the lustre, colour, and play of prismatic colours of 

 demantoid would no doubt render it one of the most highly prized of precious stones, its 

 lack of hardness not being suflicient to seriously affect its application in this direction. 



Other Gem- Varieties of Garnet. 



There are still to be mentioned a few other varieties of garnet which are used for 

 ornamental purposes. The brownish-green calcium-aluminium garnet known as grossularite, 

 fine crystals of which occur in the Vilui river, Siberia, is sometimes cut under the name of 

 " gooseberry-stone." A beautiful rose-pink, though rarely perfectly clear and transparent, 

 calcium-aluminium garnet occurs in large, well-developed rhombic dodecahedral crystals in 

 a finely granular limestone at Xalostoc, in the State of Morelos in Mexico, and is sometimes 

 employed as a gem. 



The black calcium-iron garnet, melanite, is used to a limited extent in mourning 

 jewellery ; it differs from all other garnets in occurring exclusively in volcanic rocks, such, 

 for example, as those of the Kaiserstuhl, near Freiburg in Breisgau, and at Frascati, in the 

 Albanian Hills, not far from Rome. 



Another calcium-iron garnet, known as topazolite on account of its similarity in trans- 

 parency and colour to yellow Brazilian topaz, occurs in well-developed, crystals in the Ala 

 valley in Piedmont. 



