440 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



■ may be present in large or in small amount and is sometimes dolomitic ; that is to say, it 

 contains magnesia, as is shown by the above analysis. 



Embedded in the calcite are to be seen numerous grains of minerals of various kinds. 

 A considerable number of these are quite or almost colourless, and consist of augite and of 

 hornblende. The remaining grains are constituted of the true lapis-lazuli substance ; they 

 impart to the mineral its colour, and to a certain extent other of its characteristic features. 

 At times they replace all other constituents, so that the whole mass is made up of them 

 almost entirely, while at other times they are distributed singly through the calcite. If 

 these grains are present in large numbers the colour of the stone is deep and full, and 

 according as they are distributed regularly or iiTegularly through the mass the latter i.'^ 

 uniformly coloured, patchy, or streaky. The colour of the stone as a whole depends upon 

 the colour of these grains, and these may be blue, green, or violet, while the blue grains 

 vary in shade from a deep, intense blue to one which is almost colourless. In outline these 

 coloured inclusions are nearly always rounded, angular, or ragged with numerous indentations 

 and projections. Sometimes, however, they take a regular crystalline form identical with 

 that of the larger ci'ystals mentioned above, that is to say, the' form of a rhombic 

 dodecahedron ; such crystals, therefore, also belong to the cubic system. Their reference 

 to the cubic system is supported by the fact that the majority of the grains are singly 

 refracting ; a few indeed are doubly refracting, but these are no doubt cases of anomalous 

 double refraction such as is often observed in cubic minerals. Indications of cleayage 

 parallel to the faces of the rhombic dodecahedron are also sometimes to be observed. Not 

 infrequently a certain number of the small blue grains are aggregated together in circular 

 groups. 



These constituents of lapis-lazuli, to which the colour of the mineral is due, do not 

 appear to be all of the same character. The two Swedish mineralogists, Backstrom and 

 Brogger, during the course of an important investigation into the composition of lapis-lazuli, 

 succeeded in isolating those pigment granules which differ in character. It was found that 

 one kind of these granules has the composition of the blue mineral haiiynite, which will be 

 considered later, since it is sometimes used as a gem. A second kind has the composition of 

 an artificial substance much used as a pigment and known as ultramarine. Lapis-lazuli 

 may, therefore, be considered to contain natural ultramarine, and, indeed, before the 

 introduction of artificial ultramarine this pigment was derived exclusively from lapis-lazuli, 

 and was naturally very expensive. There are also, sometimes, a few granules of still another 

 kind, differing again in chemical composition, but the two mentioned above are the most 

 important. Their composition is given by the following formulas : 



Haiiynite, 3(Na2,Ca)0.3Al203.6Si02.2(Na2,Ca)S04. 

 Ultramarine, 3Na20.3Al203.6Si02.2Na2S3. 



Each thus contains a molecule of sodium and aluminium silicate, NagO. AI, O 2SiO.„ 

 which in haiiynite has some of the sodium replaced by an equivalent amount of calcium ; in 

 addition to this a certain amount of sodium sulphate enters into the composition of haiiynite. 

 and in ultramarine some sodium sulphide. 



From the analysis given above it can be calculated that in that particular specimen there 

 was present 769 per cent, of haiiynite, 15-7 per cent, of ultramarine, and 7-4 per cent, of other 

 blue grains with the chemical composition of the mineral sodalite. This mineral, like 

 haiiynite and ultramarine, contains sodium and aluminium silicate, but the sodium sulphate 

 of the former mineral and the sodium sulphide of the latter is replaced in sodalite by sodium 

 chloride. The proportions in which these three pigments are present are ver\' variable; 



