62 GENERAL CHARACTERS OF PRECIOUS STONES 



in the dichroscope are due to various combinations of the principal colours. The axial 

 colours of each precious stone will be given below along with their special descriptions. 



The detection of dichroism in a coloured stone is a simpler matter than the observation 

 of double refraction, and the dichroscope is a less expensive instrument than the polariscope, 

 hence the former is the more often used. The polariscope may also be used to detect the 

 presence or absence of dichroism by removing the Nicol's prism, n (Fig. S7), and placing the 

 stone to be tested on the object-carrier. If the stone is not dichroic, as for instance spinel, 

 there will be no change of colour as it is rotated with the object-carrier. If, on the contrary, 

 a dichroic stone, such as ruby, is examined in this way, the colour will be seen to change as 

 the stone is rotated, varying between two extremes ; the change from one extreme to the 

 other will occur four times during a complete rotation of 360°. These two colours are 

 identical with those seen when the stone is examined in the same position in a dichroscope ; 

 the advantage of the latter instrument lies in the fact that the two colours can be seen side 

 by side, and thus small differences between them more easily detected. Just as in the use 

 of the dichroscope, several observations must be made before the absence of dichroism 

 can be considered to have been conclusively proved. As explained before, in using the 

 polariscope the portion of the field occupied by the stone may remain dark owing to 

 the total reflection of light within the stone. This can be avoided as before, by placing 

 the stone in a certain position or immersing it in a strongly refracting liquid. Care must 

 also be taken that all side light, which might be reflected from the surface of the stone, 

 is screened off with the hand, or by means of a paper tube placed around the stone. 



As we have already seen, dichroism is a character of which important use can be made 

 in identifying precious stones, and in distinguishing them from each other and from glass 

 imitations. Moreover, its observation does not necessitate a mounted stone being removed 

 from its setting, which would often be necessary in the observation of other optical 

 characters. 



Dichroism is a character of precious stones which is important also from other points of 

 view, such as that of the lapidary. A stone in which dichroism is strong must be so cut 

 that the rays of light received by the observer have passed through the stone in a direction 

 such that they will appear of the finest colour possible. Such a stone as cordierite, for 

 example, must be so cut that the dark blue colour is prominently brought out, which will 

 give a far more pleasing effect than if the light blue or yellowish-grey were predominant. 

 The beauty and, consequently, the value of two pleochroic stones of the same size 

 and quality will accordingly depend upon the manner in which they are cut, and hence a 

 knowledge of the dichroic properties of stones is of importance to the gem-cutter. 



Dichroic stones are sometimes cut and mounted in a manner which will bring out this 

 character as prominently as possible. With this object in view a cube is fashioned out 

 of the stone, the faces of which are perpendicular to the directions in which the greatest 

 differences in colour are exhibited. Such cubes are pivoted at one corner, so that on being 

 turned round the different colours will successively come into view. Cordierite, andalusite 

 and other stones are cut and mounted in this way, as will be explained in more detail later. 



7. Special Optical Appearances and Coloue Effects. 

 t 



In this section we shall consider certain optical peculiarities and colour effects of a 



special and more or less abnormal kind ; these features are not shown by every specimen of 



a particular mineral species, but only by isolated examples. These appearances are 



governed by the ordinary laws of reflection and refraction of light, and are due to the 



