72 BULLETIN 118, UNITED STATES NATIONAL MUSEUM. 



LIST OF SPECIMENS. 

 MORAVIA. 



Two stones, step-brilliant and brilliant, elliptical and rectangular girdles; 

 dark green; 23.11 and 4.95 carats; 24 by 17 by 11 and 13 by 10 by 7 mm. . . No. 681 



Moonstone. — See under Feldspar. 

 Moss Agate. — See under Chalcedony. 

 Nephrite. — See under Jade. 



OBSIDIAN. 



Synonyms or varieties.- — Rhyolite glass, volcanic glass, hyaline 

 rhyolite. 



Composition. — Glassy volcanic rocks of variable composition, chiefly 

 silicates of aluminum, iron, calcium, and the alkalies. 



Crystallization. — None. 



Color.— Various shades of black, brown, or red. 



Luster. — Vitreous; transparent to translucent. 



Hardness. — 5.5. 



Specific gravity. — Varying with composition, but mostly around 

 2.5. 



Optical properties. — Refractive index variable, but mostly about 

 1.6; optically isotropic. 



The resemblance to ordinary glass usually distinguishes this mate- 

 rial from other stones, but its isotropic character and low index of 

 refraction are usually confirmatory tests. Obsidian is of common 

 occurrence in many volcanic regions, where it is a result of the rapid 

 cooling of a molten magma that under different conditions might 

 have become crystalline. Artificial glasses similar in appearance to 

 obsidian can readily be prepared. The variegated and more bril- 

 liantly colored varieties have sometimes been cut, but the color is 

 rarely sufficiently attractive to give it any value other than as a 

 curiosity. 



LIST OF SPECIMENS. 



MEXICO (AZTEC OBSIDIAN MINE, NEAR REAL DEL MONTE, HIDALGO). 



Three stones, cabochon, elliptical girdle; black or dark brown with cat's-eye 

 chatoyancy; 20.33. 13.84, 6.56 carats No. 680 



UNITED STATES. 



Wyoming. 



Yellowstone National Park: 



Two stones, cabochon, rectangular girdle; brown-black; 28 by 21 by 7 



and 25 by 19 by 6 mm No. 683 



Two stones, cabochon, elliptical girdle; red-brown with black blotches; 



26 by 19 by 6 mm No. 682 



Two stones, cabochon, circular girdle; red-brown; 26 by 10 mm No. 684 



Two stones, cabochon, rectangular girdle; red-brown mottled with 



black; 22 by 18 by 10 mm No. 686 



