. L'Australian Museum magazine. L'histoire naturelle. ^J**K f. Le fonctionnement de l'opale à Lightning Ridge. [Photo.-Dr. J. C. danois. ressemblant à des grappes de raisins. Le noble, ou opales, qui est aussi relativement pure silice hydraté, est le plus connu de variété, même s'il est constaté que dans quelques zones défavorisées. Opale commune, contrairement à ce qui précède, est opaque, en raison d'im- puretés inclus, et obtient divers noms, tels que le lait, de l'opale opale prase, foie, et plusieurs autres. Le plus important et le plus esting inter- est bien sûr, les opales, qui constitue l'une des principales caractéristiques de l'Australie

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. The Australian Museum magazine. Natural history. _^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. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original work.. Australian Museum; Australian Museum. Sydney, Australian Museum

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