466 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



piedra de la hpada, meaning kidney-stone ; its use is always attended with confusion, and it 

 would be an advantage to dispense with it altogether. 



Although jadeite and chlovomelanite are apparently so similar to nephrite, chemical 

 analysis and microscopic examination show that they are really dissimilar, and that while 

 nephrite belongs to the amphibole group of minerals, jadeite and chloromelanite must be 

 included in the pyroxene group. All the essential characters of these two minerals are 

 identical; they contain the same constituents, but whereas chloromelanite contains a 

 considerable amount of iron, in jadeite there is very little of this element present. 

 Chloromelanite may, therefore, be considered as a jadeite rich in iron and of a 

 correspondingly dark colour. 



Chemically, jadeite is very similar to spodumene, and the variety of that mineral 

 known as hiddenite, the only chemical difference being that the lithium of spodumene is 

 replaced in jadeite by sodium. It is essentially a silicate of sodium and aluminium with 

 the formula Na^O.AlgOg.^SiO^ ; it always contains small amounts of other substances, 

 however, and on this account no two analyses are ever identical. It must, however, be 

 observed that the simple formula given above cannot be deduced directly from analyses of 

 chloromelanite, and this is a point which requires further chemical investigation. A 

 comparison of the two analyses quoted below, one of jadeite from Burma and the other of 

 an axe-head of chloromelanite from Department Morbihan (Brittany), reveals a close 

 correspondence in the chemical composition of the two substances. If, on the other hand, 

 these two analyses are compared with that of nephrite, it will be observed that in nephrite 

 aluminium and sodium are almost absent, that there is very little calcium either in jadeite 

 or in chloromelanite, and that jadeite contains scarcely any magnesium. 



Jadeite. Ghloromelauite . 

 (Per cent.) (Per cent.) 



Silica (SiOj) . 

 Titanium dioxide (TiO^) 

 Alumina (AI2O3) . 

 Ferric oxide (Fe203) 

 Ferrous oxide (FeO) 

 Manganese oxide (MnO) 

 Lime (CaO) . 

 Magnesia (MgO) . 

 Soda (Na^O) . 

 Potash (K2O) 



101-11 100-57 



Like nephrite, jadeite and chloromelanite may be described as very finely fibrous 

 to compact aggregates. The minerals have no regular external form ; a microscopic 

 examination of thin sections shows them to consist of an irregularly interwoven mass of 

 fine fibres. It is on this structure that the extreme toughness and compactness of the 

 minerals, as well as their uneven splintery fracture, depends. 



Transparent specimens of these minerals are never seen ; very thin splinters are at mpst 

 only translucent or feebly transparent. The lustre on fractured surfaces is slight, but is 

 much heightened by polishing, being then somewhat inclined to be greasy ; the same lustre 

 may be seen on many rounded, water-worn pebbles. The substance of jadeite in itself is 

 colourless, and many natural specimens are quite or almost white. More frequently the 

 mineral shows a tinge of rose-red or some light shade of colour, such as pale grey, greenish- 

 white, bluish-green, leek-green, or apple-green. Some varieties are white, with more or less 



