CATALOGUE OF GEMS AND PRECIOUS STONES. 103 



Grand Marais, Cook County — Continued. 



Three stones, one cabochon, rectangular girdle; two double cabochon, 

 elliptical girdle; mottled white to red brown, dark green and blue; 

 27.52, 13.83, and 7.73 carats; 22 by 18 by 9, 19 by 14 by 10, and 16 

 by 12 by 7 mm. Isaac Lea collection (fig. 6, pi. 7) No. 430 



Four stones, cabochon, elliptical girdle; mottled various colors; 5.97, 

 5.95, 5.76, and 4.49 carats; 14 by 11 by 6, 15 by 1 1.5 by 5, 13 by 11 by 

 7, and 18 by 8 by 5 mm. Isaac Lea collection No. 1261 



Thulite. — See under Zoisite. 

 Tiger Eye. — See under Crocidolite. 



TITANITE. 



Syn on ym. — Sphene . 



Composition. — Calcium titanium oxy-orthosilicate, Ca(TiO)(Si0 4 ). 



Crystallization. — Monoclinic; usually wedge shaped. 



Color.— Gray when pure, but usually colored yellow, green, or 

 brown by iron, manganese, or other metals not as yet recognized. 



Luster. — Adamantine ; transparent. 



Hardness. — 5.5; not very durable. 



Spi cific gravity. — 3.50 ± 0.05. 



Optical properties. — Mean refractive index, 1.95; double refraction 

 very strong, 0.15; optically biaxial, positive. Slightly pleochroic. 



The high index of refraction renders titanite very brilliant, the 

 play of colors, in fact, approaching that of the diamond. It is best 

 distinguished by the optical properties. 



Titanite occurs in metamorphic rocks, both limestone and schist, 

 as well as in veins, from which the best quality stones are obtained. 

 It is usually cut facetted ; the inferior hardness renders it somewhat 

 unsatisfactory as a precious stone. 



LIST OF SPECIMENS. 

 TYROL (ZILLERTHAL). 



Brilliant, rectangular girdle; brown-green ; 6.177 carats; 12 by 10.5 by 7.5 mm. No. 548 

 Step-brilliant, rectangular girdle; green-yellow; 2.68 carats; 10 by 7 by 4 mm. . No. 549 



UNITED STATES. 



New York. 

 Brewster, Putnam County: 



Step-brilliant, circular girdle; yellow-brown; 8.5 carats; 13 by 12 by 8 



mm No. 550 



Brilliant, circular girdle; yellow; 2.55 carats; 9.5 by 5.5 mm. Isaac 

 Lea collection No. 551 



Pennsylvania. 

 Bridgewater, Delaware County: 



Step-brilliant, rectangular girdle: dark green-brown; 4.33 carats; 11 by 

 8.5 by 6 mm. Gift of Dr. W. H. Forwood No. 552 



TOPAZ. 



Composition. — Aluminum fmo-orthosilicate, Al 2 (F,OH) 2 (Si0 4 ). 

 Crystallization. — Orthorhombic; habit prismatic; cleavage basal. 



