THE GARNET GROUP (DEMANTOID) S61 



rhodolite combined with its transparency •• HI!. ancy renders it an even more striking 



object by candlehght than by dayhght. ■: ustre of rhodolite is comparable with that 

 of demantoid, a green garnet from the "^Jrals ; this, together with its freedom from internal 

 flaws and inclusions, makes it when cut a very striking and beautiful gem. The chemical 

 composition of this new variety of garnet is shown by the following analysis : 



SiOj. AI2O3. Fe^Os. FeO. MgO. CaO. 



41 -59 2313 1-90 15-55 17-23 092 = 100-32 



The chemical formula which represents this composition is : 



2(3MgO.Al203.3SiO.) + 3FeO.Al20.,.3Si02 ; 



in other words, rhodolite is a combination of two pyrope molecules with one almandine 

 molecule. The specific gravity, 3-837, is more in agreement with that of pyrope than with 

 that of almandine ; on the other hand, in spite of the preponderance of the pyrope 

 molecules, an examination by Professor Church of the absorption spectrum of rhodolite 

 shows the existence of the bands which are characteristic of almandine. 



Mr. G. F. Kunz reports, on the authority of Mr. W. E. Hidden, tliat several crystals 

 of rhodolite were found, during the summer of 1901, embedded in a decomposed saprolitic 

 rock ; these crystals are of considerable size, one weighing 3-h pounds and yielding 300 carats 

 of fine red material, free from flaws and suitable for cutting. The yield of rhodolite in that 

 year was about 200,000 carats, valued at about ^4000. 



DEMANTOID. 



Demantoid is a beautiful green precious stone belonging to the group of calcium-iron 

 garnets, as is shown by the analysis quoted above. The stone ranges in colour from a fine 

 emerald-green to a brownish- or yellowish-green, and is sometimes indeed almost colourless. 

 Demantoids of two shades of colour are represented in Plate XIV. ; Fig. 9 illustrates 

 the mineral in its rough condition, and Fig. 10 shows three cut stones. The colour most 

 commonly seen is a light yellowish-green. The emerald-green variety, as shown by the 

 above analysis, contains a small amount of chromium, and the beauty of its colour is no 

 doubt due to the presence of this element. The paler green and yellowish-green stones 

 contain no chromium ; their colour, therefore, must be due to the iron which is present. 



The lustre, the brilliancy of which is heightened by polishing, is strongly vitreous 

 inclining to greasy, while the transparency and purity of the mineral is usually perfect. 

 The index of refraction and the dispersion are both high, and by artificial illumination a 

 faceted stone often shows a fine play of prismatic colours. 



Demantoid is softer than any other garnet, its hardness being only 6J, which is less 

 even than quartz. The specific gravity ranges from 3-83 to 3-85. It fuses before the 

 blowpipe to a black magnetic glass, but only when in the thinnest of splinters. Demantoid 

 differs from all other garnets in being easily and completely decomposed, even in its natural 

 condition before being fused, by acids. 



The mineral has hitherto been found only in the Ural Mountains. It was discovered 

 in the 'sixties in the form of greenish-white or almost colourless pebbles in the gold-washings 

 of Nizhni-Tagilsk. It was met with subsequently in the Sissersk (Syssertsk) district on the 

 western slopes of the Urals in the stream Bobrovka, which flows into the Chussavaya, at a 

 spot about ten versts south-west of the village of Poldnevaya, and twenty versts to the 

 south of the smelting works of Polevskoi, first as pebbles in the gold- washings and afterwards 



