COLOUR 55 



(b) Doubly Refracting Precious Stones. 



An important character of precious stones not yet touched upon is colour. The beauty 

 •of opaque and lustreless stones, such as the turquoise, depends wholly upon this character. 

 Every shade of colour is represented among minerals used as precious stones and for 

 ■ornamental purposes. As has already been mentioned, stones which are perfectly colourless 

 ■and also perfectly transparent are described as being water-clear, or of the first water. 



In the majority of cases colour is a very variable character ; there are, however, 

 ■examples amongst minerals in which the colour is a fixed and essential character, appearing 

 the same whether the mineral is in the largest masses or reduced to the finest powder. 

 Such a stone would be described as being idiochromatic, it having a colour of its own, 

 which is essential and characteristic of the mineral. As an example of an idiochromatic 

 stone, malachite, which is always green whether in mass or in powder, may be quoted. 



The majority of precious stones are, if perfectly pure, completely colourless. But this 

 purity of composition is, as a rule, not attained, and thus, owing to the admixture of 

 foreign colouring-matter, such stones occur much more fi'equently coloured than colourless. 

 The colour, being thus due to accidental impurities, may vary in different specimens of the 

 same stone, or even in different portions of the same specimen, and must therefore be 

 regarded as a non-essential character. Stones in which colour is a variable character are 

 distinguished as allochromatic, and the foreign matter, to which their colouring is due, 

 may be regarded as pigment. Different pigments give rise to differently coloured specimens 

 of the same mineral species. Such specimens show their colour best in fragments of some 

 thickness ; in very thin splinters, or in fine powder, they appear only faintly coloured or 

 even completely colourless. 



The variety of colour exhibited by quartz well illustrates the fortuitous nature of this 

 character when due to impurities. Thus, rock-crystal is transparent and water-clear quartz, 

 smoky-quartz is brown, amethyst is violet quartz, citrine is yellow quartz, green quartz is 

 known as plasma, blue quartz as sapphire-quartz, and there are still other coloured varieties, 

 with special names. Again, the mineral corundum, which sometimes occurs colourless, is 

 known as ruby when red and as sapphire when blue ; it is also found of many other colours, 

 which will be mentioned in the special description of corundum. Though diamond, in its 

 most valuable condition, is water-clear, yet specimens of every shade of colour are found. 



The range of colour shown by any one allochromatic mineral is known as its suite of 

 colours. Thus the suite of colours shown by quartz includes brown, violet, yellow, green, 

 blue, &c. ; that by corundum includes red and blue and many others. The suite of colours 

 shown by any one mineral will not usually be shown by any other ; in nearly every case 

 certain colours will be unrepresented in the suite. 



