SILICA. 139 



OPAL. 



Compact and amorphous ; also in reniform and stalactitic 

 shapes. Presents internal reflections, often of several colors, 

 and the finest opals exhibit a rich play of colors of deli- 

 cate shades when turned in the hand. White, yellow, red, 

 brown, green and gray are some of the shades that occur, 

 and impure varieties are dark and opaque. Luster sub- 

 vitreous. H=5-5— 6-5. Gr.=2"21. 



Composition : opal consists of soluble silica and 5 10 12 

 per cent of water. 



Varieties. 



Precious opal, Noble opal. External color usually milky, 

 but within there is a rich play of delicate tints. Composi- 

 tion, silica 90, water 10, (Klaproth.) This variety forms a 

 gem of rare beauty. It is cut with a convex surface. The 

 largest mass of which we have any knowledge is in the im- 

 perial cabinet of Vienna ; it weighs 17 ounces, and is nearly 

 as large as a man's fist, but contains numerous fissures and 

 is not entirely disengaged from the matrix. This stone was 

 well known to the ancients and highly valued by them. 

 They called it paideros, or child beautiful as Love. The 

 noble opal is found near Cashau in Hungary, and in Hon- 

 duras, South America ; also on the Faroe Islands. 



Fire opal, Girasol. An opal with yellow and bright hya- 

 cinth or fire-red reflections. It comes from Mexico and the 

 Faroe Islands. 



Common opal, Semiopal. Common opal has the hardness 

 of opal and is easily scratched by quartz, a character which 

 distinguishes it from some silicious stones often called semi- 

 opal. It has sometimes a milky opalescence, but does not 

 reflect a play of colors. The luster is slightly resinous, and 

 the colors are white, gray, yellow, bluish, greenish to dark 

 grayish green. Translucent to nearly opaque. Phillips 

 found nearly 8 per cent, of water in one specimen. 



Hydrophane. This variety is opaque white or yellowish 

 when dry, but becomes translucent and opalescent when im- 

 nersed in water. 



Cacholong. Opaque white, or bluish white, and usually 



Describe opal. How does it differ from qu artz in composition ? What 

 s said of the appearance and value of noble opal ? What is fiie opal ] 

 ommon opal ? 



