264 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



for the ordinary and extraordinary rays differing but slightly. In a crystal from Ceylon 

 they have been determined for yellow sodium light to be : w = 1'7690 ; e = 1'7598. The 

 dispersion produced by corundum is also small, the refractive indices for different colours of 

 the spectrum differing but slightly ; hence the mineral shows no marked flashes of prismatic 

 colours as does the diamond. 



The high index of refraction, small dispersion, and considerable hardness of corundum 

 render the colourless or pale-coloured varieties very suitable for the preparation of 

 microscopic lenses. Pritchard who, as we have already seen, made use of diamond for this 

 purpose, in 1827 constructed lenses of very pale blue sapphire ; they have never, however, 

 come into general use. Transparent corundum finds its most extensive application in 

 jewellery, the fine appearance of the stone being due to the combination of transparency 

 and high lustre with fine colour. The mineral corundum includes some of the most 

 beautifully coloured precious stones known, and we will now consider the different colour- 

 varieties of the mineral. 



Absolutely pure crystallised alumina has no colour and is perfectly water-clear. It is 

 then known as leuco-sapphire ; it is, however, but rarely found in this condition, more 

 usually showing a more or less pronounced colour. In many cases the colouring matter is 

 irregularly distributed, sometimes being aggregated in patches surrounded by colourless 

 portions of material ; not infrequently also different portions of one and the same crystal are 

 differently coloured. As has been already stated, the various colours shown by corundum 

 are due to the intermixture with the ground substance of various foreign substances, the 

 nature of which in particular cases has never been determined. The colours vary in tone 

 from light and pale to dark and intense ; stones showing pale tones of colour have been 

 described as " feminine," and those coloured more deeply as " masculine." 



The phenomenon of dichroism is always observable in deeply coloured specimens of 

 corundum, it is not seen in very pale coloured varieties, and is the more noticeable the 

 deeper the colour of the specimen. 



Several varieties of precious corundum are recognised ; these are distinguished from 

 each other by their colour, and from the jeweller's point of view differ much in importance 

 and value. The two varieties which occur with the greatest frequency are red corundum, or 

 ruby, and blue corundum, or sapphire, the latter being more abundant than the former. All 

 other coloured varieties are in comparison almost rare. They arc known by the same names as 

 certain other stones of the same colour, being distinguished from these by the prefix " oriental," 

 which is meant to signify the possession of specially noble qualities, such as great hardness and 

 fine lustre. The same prefix is often applied to ruby and sapphire themselves with the object 

 of distinguishing them from other precious stones of the same colour. Moreover, in the case 

 of sapphire, it serves to distinguish stones of the true sapphire-blue from other colour-varieties 

 of the species, to which, with the exception of ruby, the term sapphire is sometimes extended. 

 It should be remembered that the only character which the different colour-varieties of 

 corundum have in common with the stones after which they are named is their colour ; thus, 

 by the term " oriental emerald," must be understood transparent corundum of a green 

 colour. 



The different colour-varieties of corundum are tabulated below : 



Variety. Colour. 



Ruby (" oriental ruby ") . . red. 



Sapphire (" oriental sapphire ") . . . . blue. 

 Leuco-sapphire ... . . colourless. 



" Oriental aquamarine " . . . light bluish-green. 



