302 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



naked eye. The colour of chrysoberyl is unaffected by heat. Its yellowish-green tints are 

 very similar to those of chrysolite, and for this reason the mineral is often referred to by 

 jewellers as chrysolite. After the greenish-yellow, brownish-yellow tints are most frequently 

 seen in chrysoberyl. 



The transparency of the mineral is very variable. Perfectly clear and transparent 

 stones are free from the chatoyant sheen mentioned below and which is of such frequent 

 occurrence in this precious stone. When of a yellowish-green colour it is sometimes referred 

 to in the trade as " oriental chrysolite," as is also corundum of the same colour. Cloudy 

 and opaque specimens of chrysoberyl often exhibit in certain directions a peculiar chatoyant 

 or opalescent sheen similar to that of cat's-eye (quartz-cafs-eye), only usually much finer. 

 This chatoyant variety of chrysoberyl is illustrated in Plate XII., Fig. 11 ; it is known to 

 mineralogists as cymopliane, and to jewellers as chrysoberyl-cafs-eye, oriental caffs-eye, 

 Ceylonese cafs-eye, more briefly as opalescent or chatoyant chrysoberyl, or simply as 

 cat's-eye. 



The chatoyancy characteristic of cymophane and distinguishing it from ordinary 

 chrysoberyl appears as a milky, white, bluish, or greenish- white, or more rarely golden-yellow, 

 sheen which follows every movement of the stone, and is seen to best advantage when the 

 stone is cut en cabochon. Extending across the curved surface of such stones is a silvery line 

 or streak of light, especially obvious when the stone is viewed in a strong light. This 

 streak of silvery light may be more or less sharply defined ; its boundaries are usually sharp 

 and clear in small stones, while in large specimens they are often blurred and indistinct, 

 mei"ging gradually into the dark background. These latter are less highly prized than the 

 former, while specimens which show only an irregular patch of light with no sharp borders 

 are still less esteemed. The term cafs-eye should, strictly speaking, be limited to stones 

 which show a sharply defined streak of light, others being referred to as cymophane. 



The form taken by the chatoyant reflection depends partly upon the character of the 

 stone, but partly also upon the form in which it is cut ; hence the effectiveness of any 

 particular stone will be increased or diminished by judicious or injudicious cutting. 

 Generally speaking, the greater the curvature of the cut surface the greater is the effect 

 produced. With a slight curvature, the patch of opalescent light broadens out and becomes 

 less well defined, while if the stone be cut with a perfectly plane surface nothing will be 

 seen but a perfectly uniform sheen over the whole surface. 



The appearance known as chatoyancy is strictly limited to cloudy chrysoberyl, never 

 being seen in the transparent mineral, and, as a rule, the more cloudy the chrysoberyl the 

 more marked is the chatoyancy. The cloudiness of this mineral is due to the existence in 

 immense numbers of microscopically small cavities in the substance of the stone. Some 

 idea of their abundance may be obtained from the fact that in an area of ^ inch square Sir 

 David Brewster estimated 30,000 of them to be present. Just as in star-sapphires, the 

 optical effect is due to a certain definite arrangement of these cavities, and the stone must 

 be cut with due regard to this arrangement, otherwise the chatoyancy will be enfeebled if 

 not altogether lost. 



Chrysoberyl is esteemed for the brilliancy of its lustre and the brightness of its colour. 

 It shows no play of prismatic colours, but in place of this we get the chatoyant effect 

 described above. Of transparent varieties, those which are bright in colour are most sought 

 after, but chatoyant chrysoberyl, that is, oriental cafs-eye, is still more highly esteemed, its 

 value, however, varying with thej^body-colour of the stone. In this, as in all other cases, the 

 relative value of the different varieties of chrysoberyl is, of course, subject to the caprice of 

 fashion, now the transparent varieties, then the chatoyant cafs-eye being most favoured. 



