LAPIS-LAZULI 439 



feeble, and though stronger in the material from certain localities, in other cases is quite 

 dull. Even when cut and polished there is nothing approaching a brilliant lustre in this 

 stone, and the more impure and patchy the specimen the more feeble will be its lustre. 

 The lustre acquired by polishing is soon lost when the stone is in use, owing to its low 

 degree of hardness, and it then becomes dull and less pleasing to the eye. Lapis-lazuli 

 may practically be described as a perfectly opaque mineral ; only in the thinnest of splinters 

 is it somewhat translucent. 



The hardness of lapis-lazuli is rather low, being about 5| on the scale ; the mineral 

 is thus readily scratched by quartz and is even scratched by felspar, though it is itself 

 still capable of scratching window-glass. According to various determinations, the specific 

 gravity of the mineral lies between 2-38 and 2-42 ; it will, therefore, float in the lightest of 

 the test liquids, the specific gravity of which is 265, the same as that of quartz. It is 

 perhaps the lightest of any of the minerals used as gems. 



The mineral is decomposed by hydrochloric acid, and the white material, with which 

 the blue is frequently intermixed, dissolves in the acid with effervescence, thus proving it to 

 be calcite. At the same time the blue colour gradually disappears, and the whole process 

 is accompanied by the evolution of a strong smell of hydrogen sulphide, a smell like that 

 of rotten eggs. When heated before the blowpipe the mineral fuses with difficulty to a 

 colourless and rather clear, bubbly glass. 



When examined with a lens, or even with the naked eye, it is quite obvious that lapis- 

 lazuli is not a homogeneous mineral, like diamond, ruby, and other precious stones, but a 

 mixture of several substances ; and this is demonstrated still more conclusively by chemical 

 investigation, and by the examination of thin sections under the microscope. 



Analyses show that though all specimens of lapis-lazuli contain the same chemical 

 elements, yet these are not always present in the same proportion, and their relative amounts 

 may vary between wide limits. Thus the percentage of silica, an important constituent and 

 one which is always present, varies between 43 and 67, and there is just as much variation 

 in the case of other constituents. The following analysis is that of a stone from the 

 " Orient," 28'2 per cent, of calcium carbonate and 4'5 per cent, of magnesium carbonate 

 having been deducted : 



Per cent. 

 Silica (SiOo) . . . 43-2G 



Alumina (AI2O3) . 22-22 



Ferric oxide (FegOg) . . 4-20 



Lime (CaO) . . 14-73 



Soda (NajO) . . 8-76 



Sulphuric anhydride (SO3) . . . 5-67 



Sulphur (S) . . . . . . 3-16 



100-00 



A small amount of chlorine, up to about a half per cent., is also present in some 

 specimens. 



The existence of such wide variations in the chemical composition of lapis-lazuli leads 

 one to suppose that the mineral is a mixture of different substances. The appearance of 

 thin sections under the microscope confirms this assumption, for the single mineral con- 

 stituents and their relation to each other are then distinctly observable. The ground-mass 

 is usually white calcite or limestone of finely granular structure, and in this all the other 

 minerals are embedded. The presence of calcite is the cause of the white patches and 

 streaks in the mineral, and also of its effervescence, when placed in hydrochloric acid. It 



