CATALOGUE OF GEMS AND PRECIOUS STONES. 73 



Yellowstone National Park — Continued. 



Cabochon, elliptical girdle; dark gray; 11 carats; 22 by 17 by 6 mm. 



Gift of Dr. Robert H. Lamborn No. 687 



Two elliptical disks; black; 21 by 17 by 3 mm No. 685 



Oligoclase. — See under Feldspar. 

 Olivine. — See under Chrysolite. 

 Onyx. — See under Chalcedony. 



OPAL. 



Composition. — Silica, containing a variable amount of water 

 (Si0 2 +™H 2 0). 



Crystallization. — -None ; amorphous. 



Color. — Colorless when pure, but often tinted pink, blue, and even 

 black by impurities; shows a very brilliant internal reflection color 

 owing to the presence of innumerable minute laminae. 



Luster. — Vitreous; transparent to translucent. 



Hardness. — 6; by reason of this moderate degree of hardness, its 

 brittleness, and its porous nature, is not very durable under ordinary 

 conditions of wear. 



Specific gravity. — 2.1 ±0.1. 



Optical properties. — Refractive index somewhat variable because 

 of differences in water content, but chiefly about 1.40; optically iso- 

 tropic except where irregular distribution of the water content 

 causes strain phenomena, when double refraction may be observed. 



No other precious stone approaches opal in the internal color, so 

 that this property, together with the low specific gravity and index 

 of refraction, serves to identify it. 



There are many varieties to which specific names have been given. 

 The precious opal exhibits a play of delicate colors, reflecting now 

 one hue and then another. The harlequin opal presents a variegated 

 play of colors on a reddish ground and resembles the fire opal. The 

 fire opal presents red to yellow colors, with firelike reflections, some- 

 what irised on turning. Girasol is a blue-white translucent kind, 

 presenting red reflections in a strong light. Lechosos opal is a name 

 applied to those kinds showing deep green flashes of color. Hydro- 

 phane is a white or light colored opaque kind which becomes trans- 

 parent when immersed in water. Cacholong is opaque porcelain- 

 white, blue-white, pale yellow, or red. Opal agate is agatelike in 

 structure. Jasp-opal contains several per cent of iron, and is the 

 analogue in opal of jasper in quartz. Wood opal is wood silicified 

 by opal; sometimes called lithoxyle when showing a woody structure. 

 Hyalite, or Muller's glass, is either colorless and pellucid like glass, or 

 a translucent blue-white. Moss opal contains mosslike inclusions of 

 manganese oxide and is the analogue in opal of the moss agate in 



4555—22 7 



