o24 SYSTEMATIC DESCRIPTION OF PllECIOUS STONES 



Blue, ranging in shade from the finest and deepest indigo and azure to a delicate 

 si<y-blue, can also be iinparted to agate ; natural stones of this colour are, however, never 

 found in nature. The agate is first impregnated with yellow prussiate of potash (potassium 

 ferrocyanide), and afterwai'ds warmed in a solution of iron vitriol (ferrous sulphate). By 

 the interaction and oxidation of these salts, a Berlin-blue compound is deposited in the pores 

 of the stone, to which it gives its colour. This method, however, is only one out of several. 

 Artificially coloured blue agates sometimes closely resemble lapis-lazuli, and for this reason 

 are known as " false lapis-lazuli." They may be distinguished from genuine material 

 by a difference in the shade of colour, and by the fact that the latter is softer than 

 is agate. 



A green colour is imparted by impregnating the stone with chromic acid, and afterwards 

 exposing it to a high temperature. An apple-green shade like that of chrysoprase can be 

 imparted with a solution of a nickel salt. 



Agate is coloured red in the same way as is carnelian, namely, by impregnating it with 

 & solution of iron vitriol and then exposing it to heat. Agate is also frequently coloured 

 brown, as was mentioned above under sard. 



Some agates undergo an advantageous colour change by being simply heated or 

 *' burned " ; the bluish or greyish tints, for example, may thereby be changed to milk-white, 

 and the yellow and brown to a fine red. 



MALACHITE. 



Malachite is a mineral sometimes used as an ornamental stone, although it has nothing 

 to recommend it for this purpose save a fine green colour. Chemically it is a hydrated 

 carbonate of copper, having the formula H^O.SCuO.COj. The percentage composition of 

 the purest specimens is : 



Cupric oxide (CuO) .... 71-95 



Carbon dioxide (CO,) . . . 19-90 



Water (H2O) ' . . . . 8-15 



The water entering into the composition of malachite is easily driven off by heat, the 

 mineral, at the same time, losing its green colour and becoming black. When a fragment 

 of malachite is placed in hydrochloric acid, or a little acid dropped on the surface of the 

 stone, there is a brisk effervescence owing to the evolution of carbon dioxide. This reaction 

 serves to distinguish malachite in its rough condition from other green minerals of similar 

 appearance. When completely dissolved in hydrochloric acid a green solution of copper 

 chloride is obtained, and drops of this impart a magnificent blue colour to a colourless 

 flame. 



Malachite sometimes occurs distinctly crystallised, usually taking the form of small 

 needles belonging to the monoclinic system. The mineral occurs much more commonly, 

 however, in the form of compact masses of greater or less size, the crystalline nature of 

 which is indicated by the radially fibrous structure. Compact masses, \\hich are apparently 

 not crystalline, and often even quite earthy in character, are also met with. 



