DIAMOND: COLOUR 133 



examined in the polariscope : the arms of the cross are mutually perpendicular and their 

 point of intersection coincides with an enclosure in the diamond. It is clear that such an 

 appearance is due to a strain in the diamond brought about by the presence of the 

 enclosure, and that the strain will be less in portions further removed from the enclosure. 



Although the anomalous double refraction of diamond is, as a rule, but feeble, stones 

 exist in which it is comparatively strong, and which show much brighter polarisation colours. 

 This is the case in the " smoky stones " of South Africa, which, because of the great internal 

 strain in their substance, have a tendency to fall to powder for no apparent reason. A 

 parallel case is that of the drops of glass known as " Prince Rupert's drops," which also show 

 strong double refraction as a consequence of internal strain. 



There is never the slightest danger of confusing anomalous with true double refraction, 

 for a mineral with true double refraction, such for example as rock-crystal, colourless sapphire 

 or topaz, will appear much more brilliantly illuminated when examined in the polariscope, 

 and, moreover, will be uniformly light or uniformly dark over its whole surface. 



Colour. — Diamond is often regarded as the type of what a perfectly clear, colourless, 

 and transparent stone should be. It can by no means, however, be always so regarded, since 

 cloudy and opaque diamonds are actually more common than those which are clear and 

 transparent, while very great variety in colour is found in this mineral. A great number of 

 diamonds are indeed perfectly colourless, and correspond strictly to the popular conception 

 of the stone ; this number is, however, only one-fourth of the total number of diamonds 

 found ; another quarter show a very light shade of colour, while the remainder, at least 

 one-half of the total, are more or less deeply coloured. 



Perfectly colourless diamonds are, at the same time, most free from impurity. 

 Absolutely pure carbon, crystallised in the form of diamond, shows no trace of colour 

 whatever, and stones of this purity ai'e naturally highly prized. A peculiar steel-blue 

 appearance is sometimes observed in stones which combine absence of colour with perfect 

 transparency. With the exception of a few specially beautifully coloured stones of great 

 rarity, these blve-zchite diamonds are the most highly prized of all ; they are not of great 

 rarity in India and Brazil, but occur in South Africa with far less frequency. 



Any colouring-matter intermixed with the substance of a diamond imparts its colour 

 to the stone, the tone of which will be faint when the pigment is present in small amount 

 and deeper when it is present in greater amount. In all- cases the amount of colouring- 

 matter relative to the mass of the stone is extremely small. 



Investigations into the precise nature of the various colouring-matters present in 

 diamonds have seldom been undertaken on account of their difficulty and expense. There 

 can be no doubt, however, that the colouring-matter of many diamonds is of an organic 

 nature, possibly some one or other of the hydrocarbons ; in other cases the pigment is 

 probably inoi'ganic material in an extremely fine state of division. We have already seen 

 that coloured diamonds contain a small amount of ferruginous material which remains 

 behind as an incombustible ash after the diamond is burnt awav, and that with colourless 

 diamonds this is not the case. There seems sufficient grounds here for the inference that in 

 such cases the colour of the stone is due to the inorganic, incombustible, enclosed material, 

 especially as the colour is neither altered nor destroyed after exposure to high temperature, 

 which would be the case were it organic in nature. In the few recorded cases in which a 

 change of colour has been observed on strongly heating the stone, there can be no doubt as 

 to the organic nature of the colouring-matter. 



The colouring of many diamonds is so faint that an unpractised observer, unless he is 

 able to compare such a stone with an absolutely colourless diamond or to place it against a 



