374 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



opal from a blow or fall of any kind, and from contact with harder substances, such 

 as dust. 



The specific gravity also is less than that of quartz ; it lies between 1"9 and 2'3, the 

 exact value depending upon the chemical composition, that is to say, upon the amount of 

 water and impurities j)resent. 



The lustre is usually of the common vitreous type, though in some opals it may be greasy, 

 resinous, or waxy. The lustre of natural specimens is only moderately strong ; it is increased 

 by cutting and polishing, but even then is in no way remarkable. One variety only, hyalite 

 or glassy opal, is perfectly transparent ; this is clear and colourless like glass, but is rarely 

 used as a gem. Both the common and the precious varieties of opal are, as a rule, cloudy, 

 being at the best only translucent or semi-transparent. In a pure condition the mineral 

 is perfectly colourless, the tint of the coloured varieties being due to the presence of 

 impurities. In colour these varieties are usually brown, yellow, or red, of various shades ; 

 green opal is rare, black is known, and the cloudy varieties of milk-opal are white. The 

 optical refraction of the mineral is low, the index of refraction for precious opal having been 

 determined to be 1'4!4. Being amorphous, opal is, of course, singly refracting. 



When heated the mineral is easily fractured, so that it is desirable that cut stones 

 should be protected from sudden changes of temperature. The constituent water is 

 expelled below a red-heat, when the specimen, if not so already, becomes cloudy and opaque. 

 Opal fuses in the oxyhydrogen flame, but is infusible in the ordinary blowpipe flame. It 

 is attacked by only one acid, namely, hydrofluoric, but differs from quartz in being soluble in 

 caustic alkalies. 



Opal is found almost exclusively in the cavities and crevices of basaltic, trachytic, and 

 other volcanic rocks ; it is occasionally met with in serpentine, but never in rocks which 

 contain no silica. All the different varieties of opal may occur in association with each 

 other and with other minerals composed of silica, such as chalcedony and quartz, with which 

 opal often forms a more or less intimate mixture. Both opal and these other minerals are 

 in all cases alteration products of the rocks in which they occur. The silica dissolved out 

 by water circulating through the rocks, which is sometimes, as in volcanic regions, very hot, 

 is redeposited in the cavities and ci'evices of the rock when the solution cools or evaporates. 

 The silica thus deposited is at first gelatinous, but on drying it takes on the characters of 

 opal. The rounded form of the masses in which the mineral occurs is a natural consequence 

 of its mode of formation, and is similar to that of the stalactites which originate by the 

 deposition of calcium carbonate from water. Moreover, in some rocks silica has been found 

 in the soft and wet, gelatinous condition in which it was deposited, and on exposure to the 

 atmosphere has been observed to harden and dry up and eventually to become indistinguish- 

 able from opal. 



Hitherto we have considered only the characters common to all opal ; those varieties 

 which are used for ornamental purposes will now be considered individually in more or less 

 detail. 



PEECIOUS OPAL. 



The most important and valuable variety is the precious or noble opal, also known as 

 the oriental opal and as the celestial opal. The features on which depend the value and 

 beauty of other precious stones are in the opal insignificant or absent. Thus it is not 

 transparent, has no pronounced body-colour, and compared with other stones has no very 

 strong lustre and only a low degree of hardness. Its beauty depends solely upon the 



