408 



SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



A good price can be obtained for large, pure chrysolites of a good deep colour, but 

 large stones of average quality are not worth more than from 4s. to 7s. per carat. The 

 mineral was more highly prized formerly than at present ; it is now about equal in value 

 to topaz. 



CORDIERITE. 



Cordierite is also known mineralogically as dichroite and as iolite, while jewellers refer 

 to it as lynx-stone or as lynx-sapphire or water-sapphire (sapjphir cCeau). The two latter 

 terms are given to this stone on account of its blue colour, and specimens of cordierite have 

 sometimes been mistaken for, or sold as, inferior sapphires. This feature, however, is the only 

 one the two minerals have in common ; in all other respects they differ very essentially. 



The chemical composition of cordierite is expressed by the formula ■ 



H2O.4(Mg,Fe)O.4Al2O3.10SiO2 ; 



it is thus a hydrous silicate of aluminium and magnesium with part of the magnesium 

 replaced by ferrous iron. An analysis of a specimen from the " Orient," probably from 

 Ceylon, gave the following results : 



Silica (SiOg) . 

 Alumina (AI2O3) 

 Ferric oxide (FejOs) 

 Magnesia (MgO) 

 Lime (CaO) 

 Water (HjO) . 



Per cent. 

 43-6 

 37-6 

 5-2 

 !)-7 

 3-1 

 1-0 



100-2 



(^ 



^ 



Later investigations have shown that the iron in cordierite is present, not as ferric 

 oxide, as shown in the above analysis, but as ferrous oxide : it is doubtless to this constituent 

 that the colour of the mineral is due. 



Well-developed crystals of cordierite are not very commonly met with ; the mineral 

 crystallises in the rhombic system, and the crystals, as a rule, have the form of short prisms 

 with basal planes, and with or without small pyramidal faces 

 (Fig. 72). The faces of the crystals are usually rough and the 

 edges somewhat rounded. There is no definite cleavage and the 

 fracture is perfectly conchoidal. The hardness of cordierite is 1\ 

 on Mohs'' scale ; the mineral is thus slightly harder than quart/. 

 Cordierite is brittle ; it fuses before the blowpipe only with 

 difficulty, and is not attacked by acids to any appreciable extent. 

 Its specific gravity is rather variable, probably in correlation with 

 the presence of a varying amount of iron ; it ranges from 2'60 

 to 9,QQ, but both higher and lower values have been given. As 

 a rule, the specific gravity does not differ widely from that of 

 quartz, being in general a little lower. 



Only perfectly transparent specimens of cordierite are suitable for use as gems, and 

 these are less common than imperfectly transparent stones. The lustre is vitreous, but on 

 fractured surfaces is inclined to greasy. After polishing, the stone becomes appreciably 



^^- 



FlG. 72. Crystalline form 

 of cordierite. 



