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THE AUSTRALIAN MUSEUM MAGAZINE. 



Opal, the Rainbow Gem, 



By C. Anderson, m.a., d.sc. 



OPAL is composed of hydrous silica, 

 and differs from quartz, agate, 

 jasper and other siHca minerals 

 merely by the presence of a variable 

 amount of water. Unlike most 



minerals it is a colloid, that is, it shows 

 no sign of crystallization ; in fact, it 

 might be described as a silica jelly. 



There are many varieties of opal, 

 which is a mineral of conunon occur- 

 ence in nature. The purest variety is 

 called hyalite or glass-opal, which is 

 transparent or translucent, colourless, 

 and commonly forms nodular bunches 



the sapphire, ruby, emerald, and other 

 coloured gems, for, by transmitted 

 light, precious opal is colourless, milky, 

 or yellowish, but shows no "fire"; it is 

 by reflected light that the wonderful 

 play of colours is seen, green, blue, or 

 red flashing out as the stone is viewed 

 from different angles. The commonly 

 accejjted explanation of this phenom- 

 enon is as follows. As the silica jelly 

 dried it became traversed by cracks, 

 which were subsequently filled by opal 

 matter containing a different amount 

 of water, and therefore differing slightly 



_^J**K f 



The Opal Workings at Lightning Ridge. 



[Photo.— Dr. J. V. Danes. 



resembling clusters of grapes. The 

 noble, or precious opal, which is also 

 comparatively pure hydrous silica, is 

 the best known variety, although it is 

 found in only a few favoured spots. 

 Common opal, unlike the foregoing, is 

 opaque, by reason of included im- 

 purities, and gets various names, such 

 as milk opal, prase, liver opal, and 

 several others. 



The most important and most inter- 

 esting kind is, of course, the precious 

 opal, which is one of the characteristic 

 Australian gems, and is considered by 

 many to be the king of gcmstones. 

 The origin of the gorgeous play of 

 colours, to which the o]>al owes its 

 beauty and its value, is still in doubt. 

 The colours are not caused by any 

 pigment in the stone, as is the case in 



in its refractive effect on light. The 

 result of this lack of homogeneity is 

 that each crack acts on light like a soap 

 film, or a thin layer of oil, and produces 

 a similar effect. 



THE OLDEST OPAL MINES. 



The o]3al mines of Czerwenitza, 

 formerly in Northern Hungary, now 

 included in Czecho-Slovakia, were until 

 recently the only important source of 

 the gem, and they have been worked 

 since very ancient times ; the opals 

 known to the Romans, by whom they 

 were highly prized, probably came from 

 there. The workings at Czerwenitza 

 are now very extensive, the under- 

 ground galleries being several miles in 

 length . 



The opal occurs here in an igneous 



