56 GENERAL CHARACTERS OF PRECIOUS STONES 



In minerals in which the lustre is other than metallic, a group which comprises nearly 

 all pi-ecious stones, eight principal colours may be recognised for the purposes of descriptive 

 mineralogy ; these colours are white, grey, black, blue, green, yellow, red, and brown. 

 Intermediate colours may be described by terms compounded of the names of the eight 

 principal colours ; as, for instance, reddish-white, greenish-blue, bluish-black, &c. The 

 different shades or tints shown by each of the principal colours are indicated by a 

 descriptive prefix ; as, for example, sulphur-yellow, grass-green, indigo-blue, smoke-grey, 

 carmine-red, &c. The colour of a mineral can be judged more correctly by observing it 

 close to the eye, when small differences in colour will be more apparent. 



The character of the colour shown by a mineral depends partly on the lustre and 

 transparency of the specimen ; it may be described by terms in use in ordinary language, 

 such as lively, warm, fresh, dull, delicate, soft, dirty, dusky, &c. The intensity of the 

 colour shown by a mineral also varies in different sipecimens ; it may be described as deep or 

 dark, when approaching to black ; high or full, when pure and intense ; light, when 

 approaching to white; finally, as pale, when more nearly approaching to white. In 

 speaking of some precious stones, for example the ruby, it was formerly the custom to 

 describe specimens with a deep or full colour as " masculine," and those with a lighter 

 colour as " feminine." These terms have now, however, fallen into disuse. 



Intensity of colour depends on the amount of colouring-matter present ; the greater 

 this is, the deeper will be the colour of the stone. When the pigment of a stone is 

 distributed equally throughout its mass, the stone will be uniformly coloured. If, on the 

 contrary, the pigment is present in some parts and absent in others, or present in varying 

 amounts in different parts of the stone, the latter will show corresponding differences in 

 colour. 



One and the same stone may be differently coloured in different parts owing to the 

 presence of different pigments ; thus sapphire often shows blue spots or patches on a 

 colourless background, and amethyst may show violet areas also on a colourless background. 

 The irregular distribution of colour in such stones detracts considerably from their beauty ; 

 specimens of precious stones in which the colour is intense and distributed with perfect 

 uniformity are therefore specially valuable. 



The distribution of colour in any one kind of stone is sometimes remarkably constant, 

 appearing repeatedly in a large number of specimens. Thus in the four-sided columns of 

 diopside from the Zillerthal in the Tyrol, which are sometimes used as gems, one end is 

 colourless and the other of a fine, dark, bottle-green coloui'. In the same way the hexagonal 

 prisms of red, green, or almost colourless tourmaline from Elba frequently have a black 

 termination (so-called negro-heads). A regular arrangement of different colours in the 

 same crystals is sometimes seen in tourmaline, as illustrated in Plate XV., Figs. 8 and 9, 

 where the central portion is red and the external portion green, the two colours being 

 sharply separated from each other. The beauty of agate is due to the arrangement of its 

 various colours in bands. The following terms are used in describing colour distribution : 

 spotted, mottled, clouded, veined, marbled, striated, banded, &c. 



Brown or black arborescent, or tree-like, markings are frequently seen in certain 

 specimens of chalcedony, and are described as dendritic markings. Stones showing such 

 markings are known as dendrites. They are cut and polished with the object of bringing 

 out the markings as prominently as possible (Fig. 89). Dendrites, among which is moss- 

 agate with its peculiar and moss-like distribution of green colouring-matter, will be further 

 considered in dealing with opal, chalcedony, &c. 



The various pigments to which the colouring of precious stones is due may be 



