DIAMOND: OFIICAL CHARACTERS 131 



of metals. The same metallic lustre is seen on the facets of a perfectly transparent stone 

 when light falls upon it at a very small inclination, the light being reflected from the facet 

 in such a way that the latter has the appearance of highly polished steel. This phenomenon 

 can be observed by placing the stone with a perfectly smooth facet close to the eye and 

 inclining it towards the light, until, in a certain position, the metallic reflection becomes 

 evident. 



Perfect adamantine lustre, in all bodies which possess it, is combined with perfect 

 transparency, very strong refraction, and marked dispersion of light. All substances 

 including diamond itself, which possess adamantine lustre are thus also characterised by 

 strong refraction and dispersion of light ; and, conversely, all substances possessing the two 

 latter qualities will be found to exhibit adamantine lustre. Not only the quality but the 

 intensity of the lustre shown by a stone depends upon the strength of its refraction of light ; 

 light rays falling obliquely upon the surface of a stone will be the more completely reflected 

 the higher its index of refraction is. Thus diamond, having a higher index of refraction, 

 will reflect more rays of light from its surface, and will therefore show a stronger lustre than 

 will a substance having a lower index of refraction. The qualities of lustre and brilliancy 

 are known collectively as the " fire " of a stone. It will be evident from what has been 

 said that the " Are " of diamond is specially fine. 



Kefractiou of light. — Diamond, like all other substances which crystallise in the 

 ■cubic system, is singly refracting. A ray of light incident obliquely upon the plane face 

 of a diamond is propagated in the substance of the stone as a single ray, the direction of 

 which, however, differs from that of its path in the surrounding medium. This difference 

 is, in diamond, very considerable, much more than in the majority of other substances; 

 in other words, the index of refraction of diamond is very high. 



The power of breaking up white light into its constituent colours, that is, the 

 dispersion, possessed by diamond is likewise very marked. The blue rays of light 

 undergo a much greater refraction when passing into diamond than do the red rays ; 

 hence the spectrum produced by a prism of diamond is very long, the red and blue ends 

 being widely separated. The various colours into which white light, in passing through 

 a cut diamond, is broken up are widely separated and distinctly perceptible; hence the 

 beautiful play of brilliant, prismatic colours upon which so much of the beauty of diamond 

 depends, and which differentiates it so markedly from other colourless stones, such as 

 rock-crystal, topaz, colourless sapphire, &c., which have a lower dispersion and consequently 

 .a less beautiful play of colours. This subject has, however, been fully dealt with above 

 in the section devoted to the consideration of the passage of light through a cut stone. 



The action of every diamond upon light is not absolutely identical. A satisfactory 

 explanation of the small differences which exist cannot, however, at present be given. The 

 fact that one stone has a finer appearance than another is probably due to slight differences 

 between them in the refi-active index and in dispersive power. In respect of play of 

 prismatic colours Indian diamonds rank highest. Next to these we may place Brazilian 

 stones from the district of Diamantina in the State of Minas Geraes, and from the 

 Canavieiras mines in the State of Bahia. Relatively inferior to these, but yet with a 

 fine play of prismatic colours, are the majority of Cape diamonds. It is a remarkable fact 

 that in many cases diamonds from the Cape and from Canavieiras exhibit a finer play of 

 prismatic colours in artificial light than in daylight, which is the reverse of what is usually 

 the case. 



The refractive power and the dispersion of diamond are both given by the values of 

 the refractive indices for different coloured rays. These values give the strength of 



