them to harden or "season," as it is 

 called, for some time after quarrying, 

 before they are polished. 



Opal as a mineral is quite common, so 

 that no one need suppose because he 

 has specimens labeled ''opal" in his col- 

 lection that he has as many precious 

 stones. It occurs in many varieties, and, 

 especially if it contains foreign matter, 

 in many colors. Nearly all silica de- 

 posited by hot waters is in the form of 

 Opal, so that the geysers of Yellowstone 

 Park build up cones of Opal and fall 

 into Opal basins. This particular form 

 of Opal is known as geyserite, and iu 

 is often differently colored by different 

 ingredients. 



Wood is often preserved by silica in 

 the form of Opal, the siliceous waters 

 taking away the wood and replacing it 

 by Opal, grain by grain, with such deli- 

 cacy and accuracy that the structure of 

 the wood is perfectly maintained. The 

 minute shells which dialoms make con- 

 sist of Opal, and when these dead shells 

 accumulate to form deposits of some ex- 

 tent we call the powdery substance trip- 

 oli and use it for polishing silverware 

 and other metals. Then there are 

 hyalite, a variety of Opal looking like 

 transfixed water, so clear and colorless is 

 it; hydrophane, a translucent variety 

 which sticks to the tongue and becomes 

 more nearly transparent when soaked in 

 water; cacholong, a porcelain-like va- 

 riety, and menilite, a concretionary va- 

 riety. 



Common Opal varies from transpar- 

 ent to opaque, being most often trans- 

 lucent and sometimes exhibiting the 

 peculiar milkiness of^ color which we 

 call opalescence. It has sometimes a 

 glassy, but often a waxy luster, the latter 

 when pronounced giving rise to the va- 

 rieties known as wax Opal and resin 

 Opal. When Opal has the banded struc- 

 ture of agate it is known as Opal-agate ; 

 when it has the color of jasper as jasper 

 Opal, and when that of chrysoprase as 

 prase Opal. But none of these varieties 

 are used in any quantity as gems. This 

 distinction is reserved almost wholly for 

 the variety known as noble or precious 

 Opal. This is Opal which exhibits a 

 play of colors. No essential chemical 

 or physical distinction between noble 



Opal and other varieties is known. In 

 a large vein of Opal portions will ex- 

 hibit the play of colors and the re- 

 mainder will not, but why the difference 

 has not yet been determined. Neither 

 can the origin of the varied coloring; 

 i. e., the iridescence, be determined. 

 Some regard it as due to interspersed 

 layers containing different percentages of 

 water, which break up the rays of light 

 somewhat as a prism does, while others 

 think that minute cracks and fissures 

 through the stone furnish surfaces from 

 which the rays are reflected in different 

 colors back to the eye. Some Opals 

 which are dull and lusterless when dry 

 exhibit considerable play of color when 

 immersed in water, and this fact seems 

 to favor the first theory of the cause of 

 the iridescence, but the subject is not 

 understood. The character of the play 

 of colors differs in different Opals, and 

 this gives rise to different varieties. The 

 true noble Opal has the color quite uni- 

 formly distributed. When the color ap- 

 pears in flashes of red, yellow, etc., the 

 stone is known as fire Opal; if blue as 

 girasol, and if chiefly yellow as golden 

 Opal. When the patches of color are 

 small, angular and uniformly distributed 

 it is called harlequin Opal, and if these 

 are long and somewhat parallel, flame 

 Opal. These colors are not, of course, 

 inherent in the stone, its color vary- 

 ing from colorless to opaque white. 

 The black Opals sometimes seen and 

 highly prized by some are usually arti- 

 ficial, and are made by soaking ordinary 

 Opals in oil and. then burning oil on 

 them. The brilliancy of the stone is 

 thus increased, but it is usually fragile 

 and liable to lose color. 



Any Opal will lose its play of colors 

 on being heated too highly, hence pos- 

 sessors should avoid subjecting them to 

 more than ordinary heat. It is the va- 

 riety and brilliancy of their changing 

 colors which give to Opal nearly all its 

 desirability as a precious stone, for, as 

 has often been remarked, the qualities 

 of hardness, transparency and rich body 

 color which give to most other gems 

 their value are lacking in the Opal. But 

 together with the beauty of its chang- 

 ing colors Opal possesses an advantage 

 over all other gems in that it cannot 



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