46 



GENERAL CHARACTERS OF PRECIOUS STONES 



regarded as the mean path for the several colours, or, more correctly, as the path which 

 would be taken by monochromatic light within the stone. Other rays of light entering the 

 front and side facets of the stone will be refracted and totally reflected in the same manner, 

 and will therefore follow a path very similar in direction to the one shown in Fig. !20. 

 The whole stone will therefore appear to be full of light, and will emit flashes of rainbow 

 colours. 



The many beauties of the diamond can be traced back to the optical characters of the 

 stone ; its high index of refraction causes a large proportion of the light which enters the 

 front facets of a suitably cut stone to be totally reflected from the back faces, while from 

 its high dispersive power results a wide separation of the rays of difi^erently-coloured light, 

 and, in consequence, a fine play of prismatic colours. These features of a cut diamond are 

 specially noticeable when the stone is contrasted with another colourless stone, cut in 

 the same manner, for example, rock-crystal. The latter appears in comparison dull and 

 dead, owing to the fact that it possesses neither the high index of refraction nor the 

 great dispersive power of the diamond. The highly refractive and dispersive glass called 

 strass, when cut in the form of a brilliant, may, however, closely resemble a diamond in 

 these characters. 



From what has been said above, it is easy to see that the cutting of a stone is a very 

 important factor in developing the potential beauty with which its optical characters endow 

 it. A diamond cut in the form of a good brilliant, far exceeds in play of colour and general 

 brilliance a similar stone cut in any other form, such, for instance, as a rosette (rose-cut), 

 which does not fully utilise the optical characters of the stone. 



•D 



4. Double Refkaction of Light. 



Hitherto we have considered only those substances in which a single refracted ray 

 corresponds to a single incident ray. There are, however, many bodies, including many 

 precious stones, which have the property of splitting a single incident 

 ray of light into two refracted rays which are propagated in their 

 substance along paths differing slightly in direction. 



In Fig. 21 the ray AB, travelling in air L, is incident upon 

 the surface MN of the stone S at B, where it is split into the two 

 refracted rays BO and BE, inclined to one another at a very small 

 angle OBE, which never exceeds a few degrees. 



Bodies which behave towards light in this way are described 

 as being doubly refracting or birefringent, in contradistinction 

 to the singly refracting bodies hitherto considered. Substances 

 exhibiting the phenomenon of double refraction may also be described as optically 

 anisotropic, while those which exhibit single refraction are described as being optically 

 isotropic. 



As far as regards the transparency, lustre, colour, and play of colour of a stone — those 

 characters in short which affect the beauty of the stone — it is unimportant whether the light 

 within it is singly or doubly refracted. 



The phenomenon of double refraction can be easily observed by the aid of special 

 instruments. The detection of its presence or absence is a valuable aid in identifying and 

 discriminating precious stones in the cut condition. Thus by the aid of an appropriate 

 instrument we can decide whether a certain red stone is a doubly refracting ruby or a singly 

 refracting spinel, two stones which, though very similar in appearance, are very dissimilar in 



Fig. 21. Double refrac- 

 tion of a ray of light. 



