518 REPORT OF NATIONAL MUSEUM, 1900, 



often inclining to resinous, and occasionally to pearly. Color, white, 

 yellow, red, brown, green, gray (the dark color arising from foreign 

 admixtures); sometimes having a brilliant play of colors by reflected 

 and refracted light. This play of colors is not due to the presence of 

 colored substances as constituents of the mineral, but to a phJ^sical 

 condition of the specinicn resulting from a multitude of fissures, the 

 sides of which are minutely striated, and which cause a diflraction and 

 decomposition of the light which falls upon them. The size of these 

 striations and fissures influence the color and its distribution within 

 the mineral; occasionally the patches of color are of a uniform size; 

 again, the}^ may be irregular. Certain specimens may show a predom- 

 inance of one set of colors, as red and orange; others ma}' show chiefly 

 green and blue tints. 



There are many varieties of the opal 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 jire opal presents hyacinth red to honey yellow colors, 

 with fire-like reflections, somewhat irised on turning. Gtrasol is a 

 bluish white translucent kind, presenting reddish reflections in a 

 strong light. Lechosos opal is a name applied to those kinds showing 

 deep green flashes of color. Ilydrophane is a whitish or light colored 

 opaque kind which becomes transparent when immersed in water. 

 Cacliolong is an opaque porcelain, bluish, yellowish, or reddish white 

 variety. 6>^a^ «^6«^e is agate -like in structure. c^Tas^op^^ contains sev- 

 eral per cent of iron, and is the analogue in opal of jasper in quartz. 

 Wood opal is wood silicified by opal; sometimes called llthoxyle when 

 showing a woody structure. (Plate 4.) Hyalite, or Mullerh glass, is 

 either colorless and pellucid like glass, or a translucent bluish white. 

 Moss opjol contains moss-like inclusions of manganese oxide and is the 

 analogue in opal of the moss agate in quartz. Tabashee7' is an amor- 

 phous opal-like silica deposited within the joints of bamboo; it absorbs 

 water and becomes transparent like hj^drophane. 



The opal occurs filling seams, cavities, and fissures in igneous rocks, 

 etc. It occurs also embedded in limestone and argillaceous beds. It 

 consists essentially of silica in the soluble or gelatinizing condition, 

 but often combined with insoluble silica, and is more or less hydrous. 



ORTHOCLASE. 



ADULARIA — AVENTURINE — MOONSTONE — PERTHITE. 



Orthodase, common feldspar, occurs in monoclinic crystals, often 

 prismatic in habit, and massive, coarsely cleavage to compact, and 

 flint-like or jasper-like. Cleavage perfect in one direction, less so in 

 another. Hardness, 6. Specific gravity, 2.4 to 2.6. Luster, vitreous, 

 sometimes inclining to pearly on the face of a perfect cleavage, and 



