CATALOGUE OF GEMS AND PRECIOUS STONES. 29 



LIST OF SPECIMENS. 



CANADA (THETFORD). 



One double cabochon, four cabochon, circular girdle; black, polished; 1.435 

 grams; 6 mm. average diameter. Isaac Lea collection No. 679 



CHRYSOBERYL. 



Varieties. — Alexandrite; cat's-eye or cymophane. 



Composition. — Beryllium aluminate, BeAl 2 4 . 



Crystallization. — Orthorhombic ; habit tabular. 



Color. — Pale yellow, green-yellow, yellow-green, dull green, and 

 rarely brown-yellow or orange-brown; in artificial light, red, espe- 

 cially in the emerald green variety alexandrite; pleochroic, green to 

 red, in deep colored varieties. 



Luster. — Vitreous ; in cat's-eye, silky. 



Hardness. — 8.5; a very durable stone. 



Specific gravity. — 3.60 ±0.1. 



Optical properties. — Mean refractive index 1.75; double refraction 

 moderate, 0.01; optically biaxial, positive. 



The variety cymophane, the true cat's-eye, shows minute lami- 

 nations which yield a peculiar luster. Alexandrite shows with the 

 microspectroscope a strong absorption band in the yellow and can be 

 recognized by the red color in artificial light; pale colored varieties, 

 including cat's-eye, can be distinguished from similar minerals by 

 the high specific gravity and by optical properties. The mineral 

 occurs in pegmatite and in mica gneiss. The chief commercial 

 sources of chrysoberyl are Brazil and Ceylon. The variety alexan- 

 drite comes chiefly from the Takovaya stream in the Ekaterinburg 

 district of Russia, though it has more recently been found in Ceylon. 

 The mineral "was discovered in Russia on the very day on which 

 the coming of age of the Czarevitch Alexander Nicolajevitch, after- 

 wards Czar Alexander II, was celebrated, and in his honour it was 

 named by the Finnish mineralogist Nils von Nordenskiold." 



Stones resembling chrysoberyl have been produced artificially, 

 but their complete identity with the natural mineral remains to be 

 proved. The peculiar property shown by alexandrite of changing 

 color in artificial light renders it rather highly prized as a gem; 

 cat's-eye is usually cut cabochon so as to bring out the luster. 



LIST OF SPECIMENS. 

 BRAZIL. 



Brilliant, square girdle; pale yellow-green; 6.329 carats; 11 by 7.5 mm No. 640 



Brilliant, elliptical girdle; grccn-yellow; 2.5 carats; 10 by 7 by 5 mm No. 641 



Trap-brilliant, elliptical girdle; green-yellow; 1.84 carats; 8 by 7 by 4 mm.. No. 642 



Table, rectangular girdle; green-yellow; 0.99 carat; 7 by 5 by 3 mm No. 643 



Step-brilliant, elliptical girdle; green-yellow; 0.965 carat: 7.5by6by3 mm. No. 644 



Table, square girdle; pale green-yellow; 0.745 carat; 5 by 3 mm No. 645 



Step-brilliant; irregular girdle; green-yellow ; 0.495 carat; 5 by 3 mm No. 647 



Step-brilliant, pentagonal girdle; green-yellow; 0.4S2 carat; 5 by 5 by 3 mm. No. 648 



