266 SYSTEMATIC DESCRIPTION OF PHECIOUS STONES 



allowed to cool have been already described. During cooling the ruby becomes first white 

 and then green, finally regaining its original red colour, so that in this stone the colouring- 

 matter is neither permanently changed nor destroyed by exposure to high temperatures. It 

 is otherwise, however, with the sapphire, for this gem at a high temperature loses its. 

 beautiful blue and takes on a dull grey colour. 



The red colouring-matter of the ruby is therefore certainly not organic in nature, as 

 seems to be the case with those gems which lose their colour on heating. It is more likely 

 to be some compound of chromium, an element whose presence has been detected in the 

 analysis of some rubies. That the colouring of the ruby is due to chromium is also, 

 suggested by the fact that the colour of the so-called " ruby " glass is obtained by adding a. 

 small amount of chromium oxide to the other constituents of the glass. The same substance 

 is also used by M. Fremy for the production of the red colour of his artificially prepared 

 rubies. Some of the crystals produced by this investigator were partly red and partly blue,, 

 resembling in this respect certain natural rubies which occur rarely in Burma. 



The dichroism of deeply coloured rubies is very noticeable ; with the exception of 

 stones of very pale colour, a difference in the colour of every ruby can be observed when 

 viewed in different directions. On looking through a dark-coloured crystal of ruby, such as. 

 is illustrated in Fig. 53, a — d, in a direction perpendicular to the basal planes, it will 

 appear of an intense red colour, either pure red or with a slight tinge of violet. If, however,, 

 the light received by the eye has passed through the crystal in any direction perpendicular 

 to a prism face or edge, the stone will appear much lighter in colour. On allowing the light 

 which has passed through the crystal in this direction to enter the dichroscope, the two- 

 images, in that position of the instrument in which the greatest difference in colour is- 

 shown, will be one light, and the other dark redr usually tinged with violet. In all other 

 directions in which the light may travel, with one exception, the two images will be more or 

 less differently coloured. This exceptional direction is perpendicular to the basal planes 

 and coincides with the direction of the optic axis. Along this direction the crystal is singly 

 refracting, and the two images seen in the dichroscope are of the same deep red colour as 

 the crystal appears when viewed in this direction without the intervention of the dichroscope.. 

 The dichroism of the ruby affords a means whereby it may be distinguished with certainty 

 from other red stones, such as spinel and the different varieties of garnet, which crystallise- 

 in the cubic system, and thus being singly refracting can show no dichroism. 



The fact that the colour of the ruby varies with the direction in which it is viewed,, 

 makes it necessary that the form of the cut gem should have a certain definite relation to- 

 that of the crystal in order to obtain the finest colour-effect. The plane of the largest 

 facet of the cut stone, namely, the table, must coincide as closely as possible in direction- 

 with the basal planes of the crystal in order to obtain the greatest depth in colour of which 

 the stone is capable. The greater the angle at which the table is inclined to the basal 

 plane of the crystal the poorer will be the colour-effect produced, and when the table is- 

 perpendicular to the basal plane, and therefore parallel to the prism faces of the crystal, the 

 minimum colour-effect is the result. 



Some rubies show on the basal plane, or still more plainly on a cut and polished curved 

 surface approximating to the basal plane in direction, a six-rayed star of glimmering 

 reflected light. Such stones are known as star-rubies, or asteriated rubies, sometimes also as- 

 ruby cat's-eye. The appearance is similar to that seen in the star-sapphire, but, as a rule, 

 less marked ; it will be therefore considered in greater detail under sapphire. 



Value. — A clear, transparent, and faultless ruby of a uniform deep red colour is at the- 

 present time the most valuable precious stone known. Except in ancient times, it is. 



