CATALOGUE OF GEMS. 517 



The percentage composition of olivine, though the iron oxide may 

 be more or less replaced b}' magnesia, is, approximately: Silica, 41; 

 magnesia, 50; iron oxide. 9. 



OCTAHEDRITE. 



Octahedrite crystallizes in the tetragonal s3^stem, commonly on octa- 

 hedra. Its hardness is nearly 6, its specific gravity -ii.sa, and its color 

 includes the several shades of brown, passing into deep blue or black; 

 ^sometimes greenish ^^ellow to pale green. Fine blue crystals of this 

 mineral from Brazil are so remarkabh^ brilliant as to be mistaken for 

 diamonds. 



One hundred parts of octahedrite contain; Titanium, 60; oxygen, 40. 



ODONTOLITE. 



BOXE TURQUOISE — FOSSIL TURQUOISE. 



Odontolite is a fossil ivory or bone, colored sky blue or green by 

 phosphate of iron. Tt resembles the turquoise in color, but may read- 

 ily be identified by its bony structure. 



OLIGOCI.ASE. 



HELIOLITE SUNSTOXE. 



Oligoclase. one of the feldspars, occurs massive and in triclinic forms 

 crystals somewhat rare. Its cleavage is perfect in one direction, less 

 so in another. Fracture uneven. Hardness, 6. Specific gravity, 

 2.65 to 2.67. Luster, vitreous to somewhat pearly. Color, grayish, 

 reddish, greenish, and white to colorless, occasionally having a play 

 of color. Aventurine kinds, called either sunstone or heliolite, are of 

 a grayish white to reddish gi'ay color, with internal yellowish or I'ed- 

 dish reflections proceeding from disseminated crystals or flakes of iron 

 oxide. 



Fine transparent oligoclase, affording brilliant gems having a moon- 

 stone effect, is found near Bakersville. North Carolina. The ])est 

 sunstone is from Christiana fiord, Norway. The same region also affords 

 specimens having a magnificent play of color, in which grayish blue, 

 orange, and gray, with coppery reflections, are seen. 



Oligoclase occurs in granitic, syenitic, and dioritic rocks. 



One hundred parts contain, approximate v: Silica, 62.3; alumina, 

 23.5; soda. U.2. 



OPAL. 



The opal occurs in amorphous masses; sometimes in reniform, 

 stalactitic, and tuberose shapes, and encrusting. Its hardness is about 

 6.5, occasionally as low as 5.5. The specific gravity varies from 1.9 

 to 2.3, commonh' 2.1. Luster, vitreous, frequently subvitreous, 



