54 GENERAL CHARACTERS OF PRECIOUS STONES 



those diamonds known as " smoky stones," as to cause such stones to fly to pieces without 

 any apparent reason. 



Anomalous double refraction is usually, however, only feeble, and the alternations of 

 lightness and darkness exhibited in the polariscope by minerals exhibiting this character are 

 much less marked than is the case with truly doubly refracting stones. Moreover, the 

 illumination of a doubly refracting body is uniform over the whole of its area; but in the 

 case of a body showing anomalous double refraction, portions of its area will remain dark 

 dining the complete rotation of the carrier, while the portions which become alternately 

 light and dark during the rotation appear as stripes and bands, or in variously shaped 

 sectors. It is thus a comparatively easy matter to distinguish between anomalous and true 

 double refraction. 



The phenomenon now under consideration sometimes affords a means whereby a glass 

 imitation may be distinguished irom a genuine precious stone. While glass under ordinary 

 conditions is singly refracting, being an isotropic substance, unannealed glass possesses 

 anomalous double refraction. Thus if a fairly thick plate of glass be first strongly heated 

 and then suddenly cooled, internal strains will be set up, and when examined in the 

 polariscojDe it will show a more or less regular black cross, with the two arms at right 

 angles, sometimes surrounded by coloured circles. Should a supposed precious stone show 

 this or a similar appearance, the observer may regard it as conclusive evidence against the 

 genuineness of the stone. 



The refraction of a precious stone is expressed, as explained above, by a number known as 

 its refractive index. In the case of singly refracting substances there is, for monochromatic 

 light, only one refractive index, which is constant for every direction in the stone. The 

 index of refi action of a doubly refracting substance, however, varies according to the 

 direction. It is greatest in one particular direction and least in a direction at right angles 

 to this. These maximum and minimum values vary slightly for differently coloured light, 

 but are constant for monochromatic light. The greater the difference between the greatest 

 and the least values of the refractive indices of a precious stone, the greater will be its 

 double refraction, v/hich is measured by this difference. 



The number which thus expresses the strength of the double refraction of a substance 

 is constant for, and characteristic of, that substance, and could be made use of for purposes 

 of identification. Its exact determination is, however, a matter of considerable difficulty 

 and requires special and costly instruments, as well as suitable preparation of the stone 

 to be examined. This method is therefore of no practical value to jewellers. 



The refractive indices of the more important precious stones are given in the 

 following table, the values for singly refracting stones being indicated by n, and the 

 greatest and least values for doubly refracting stones by n^ and n, respectively. In both 

 cases the values apply to the middle rays of the spectrum. The strength of the double 

 refraction of each stone is indicated hy d = n^ — n„ that is, by the difference between the 

 greatest and the least refractive indices of the stone. 



& 



(a) Singly Refracting Precious Stones. 



n 

 Diamond 2'43 



Pyrope 1-79 



Almandine . 1-77 



Hessonite 1-74 



n 

 Spinel .... 1-72 

 Opal .... 1-48 

 Fluor-spar . . 1-44 



