516 REPORT OF NATIONAL MUSEUM, 1900. 



NATROLITE. 



Natrolite is occasionally used for the letter N in sentimental jewelry 

 and as a collectors' gem. The mineral occurs in groups of slender, 

 prismatic, orthorhombic crystals, in fibrous, radiated, and divergent 

 masses, in amygdules having a fibrous structure, and in compact 

 masses. Its hardness is 5.5; specific gravity, 2.2; luster, vitreous, 

 sometimes inclining to pearly or silky in the fibrous kinds; color, 

 white to grayish, yellowish, reddish, and red. 



Natrolite occurs in cavities in amygdaloidal basalt and related igne- 

 ous rocks, and in seams in granite, gneiss, and syenite. 



One hundred parts contain: Silica, 47.4; alumina, 26.8; soda, 16.3; 

 water, 9.5. 



OBSIDIAN. 



Obsidian is compact volcanic glass. Its specific gravity is 2.25 to 

 2.8. The hardness is somewhat less than that of ordinary feldspar. 

 The prevalent color of the material is black, but some of it is mottled 

 and streaked with brownish red or various shades of brown; sometimes 

 in reddish-brown spherules in a gray matrix; also green of several 

 shades to light and dark yellow. Certain fibrous specimens will afl'ord 

 the cat's-eye ray. 



The kinds used as gems are Known as: Moldavite or hottle stone ^ a 

 green-colored obsidian; nmreJcanite or mountain mahogany ^2l red col- 

 ored or banded black and brown variety, and Iceland agate^pearlylite^ 

 and sjyhaeruUte. 



OLIVINE. 



CHRYSOLITE — PERIDOT — HYALOSIDERITE. 



Olivine crystallizes in the orthorhombic sj'stem; also occurring 

 massive; compact or granular; commonly in embedded grains. Its 

 hardness is 6.5 to T. Specific gravity, 3.33 to 3.44. Cleavage dis- 

 tinct. Fracture, conchoidal. Brittle. Luster, vitreous. Color, 

 commonly olive green, sometimes brownish, grayish red, and occa- 

 sionally black. Double refraction, strong. Dichroism occasionally 

 marked, the peridot giving a straw -yellow and a green image. 



The kinds used as gems are known as: Chrysolite^ j'^ellowish green; 

 'peridot or evening emerald^ having an olive pistachio, or leek-green 

 color, of a quieter hue than that of the emerald, the approved tint 

 being similar to that seen on looking through a delicate green leaf, 

 and JtyaJosiderite^ called also JoVs tears^ a highly ferruginous variety 

 having a specific gravity as high as 3.57 and a rich olive-green color. 



Olivine is a common constituent of some eruptive rocks; not uncom- 

 mon in granular limestone and dolomite; occurring also in certain 

 schists and in ore deposits. 



