30 BULLETIN 118, UNITED STATES NATIONAL MUSEUM. 



Step-brilliant, rectangular girdle; pale yellow, 0.46 carat; 6 by 5 by 2 mm. . No. 646 

 Thirteen small gems, step-brilliant, various girdles; green-yellow; total 



■weight, 2.6 carats No. 649 



Twenty-five small gems, various cuts; green-yellow; total weight, 2.777 carats. 



Isaac Lea collection No. 662 



CEYLON. 



Brilliant, elliptical girdle; dark yellow-green; 5.1 carats; 10 by 9 by 7 mm.. No. 634 

 Step-brilliant, elliptical girdle (Indian cut); brown-yellow; 4.257 carats; 8 



by 6 by 9 mm No. 638 



Step-brilliant, rectangular girdle; dull yellow-green; 3.9 carats; 12 by 10 by 



4 mm. Isaac Lea collection No. 636 



Step-brilliant, rectangular girdle; dark green-brown; 3.09 carats; 8 by 7 by 



6 mm No. 635 



Step-brilliant, elliptical girdle; yellow-green; 1.9 carats; 7 by 6 by 5 mm. 



Isaac Lea collection No. 637 



Brilliant-rose, circular girdle; orange-brown; 1.157 carats; 6.5 by 4 mm No. 639 



CHRYSOBERYL, variety ALEXANDRITE. 



RUSSIA (URAL MOUNTAINS). 



Step-brilliant, rectangular girdle; green; 2.247 carats; 8 by 7 by 4 mm No. 691 



Step, rectangular girdle; green; 1.07 carats; 6.5 by 5.5 by 3.5 mm. Isaac 



Lea collection No. 688 



Trap, rectangular girdle; green; 0.43 carat; 5 by 4 by 3 mm. Isaac Lea 



collection No. 690 



Step-brilliant, elliptical girdle; green; 0.378 carat; 6 by 5 by 2 mm. Isaac 



Lea collection No. 689 



CHRYSOBERYL, variety CATS-EYE. 



SI AM. 



Seventeen cat's-eyes and 16 diamond chips mounted in a heavy gold ring — No. 692 



CHRYSOLITE. 



Synonyms. — Olivine, peridot. 



Composition. — Magnesium orthosilicate, Mg 2 Si0 4 . 



Crystallization. — Orthorhombic. 



Color. — Yellow-green, green-yellow, or rarely pure yellow; essen- 

 tial, due to ferrous iron replacing magnesium; very faintly pleochroic. 



Luster. — Vitreous . 



Hardness. — 6.5; a fairly durable precious stone. 



Specific gravity. — 3.40 ± 0.2. 



Optical properties. — Mean refractive index, 1.68; double refraction 

 strong, 0.03 ; optically biaxial, positive. 



Chrysolite, also known under the names olivine and peridot, is an 

 abundant constituent of many igneous rocks, as the basalts, peri- 

 dotites, and stony meteorites, but in most cases in the form of small 

 granules only. When these rocks become disintegrated, the mineral, 

 which is very refractory to weathering agencies, is set free and 

 accumulates in the residual sand. The principal American sources 

 are the Navajo Indian country of Arizona, and New Mexico. (See 

 pi. 5.) 



