by 2½ and weighing 3,000 carats, was placed in the Museum of Natural History at Vienna. The mines at Czernovitza are known to have been worked over 500 years ago, and at a more remote period they no doubt supplied the ancient world. There is little doubt, however, that the wonderful opals from Australia’s fields have eclipsed anything yet found. The White Cliffs, the Lightning Ridge, and the newer field out North West are responsible for some of the most beautiful gems that have ever been unearthed. A kangaroo hunter accidentally discovered the White Cliffs field in New South Wales over 40 years ago whilst following the trail of a kangaroo. Rich “blacks” were discovered later in the iron sandstone of Lightning Ridge (New South Wales) and the new fields North West of Tarcoola are yielding white and light varieties. Opal country is dry and dreary and the diggers deserve all they find. Sir David Brewster’s theory of the colour blends which flash from an opal is that “the stone is internally traversed with undulating fissures of microscopic minuteness upon which refraction and decomposition of light takes place. The variations in the nature of these minute cavities cause the appearance of the opal to vary considerably, and the different effects of colour thus produced are technically known as the pattern of the gem.” Hauy held that colour in the opal is caused by thin films of air which fill the interior cavities. Dr. G. F. Herbert Smith writes “that the colouration is not due to ordinary absorption but to the action of cracks in the stone. This is shown by the fact that the transmitted light is complementary to the reflected light; the blue opal, for instance, is a yellow when held up so that light has passed through it.... Opal differs,” he says, “from the rest of the principal gem stones in being not a crystalline body but a solidified jelly, and it depends for its attractiveness upon the characteristic play of colour known, in consequence, as opalescence which arises from a peculiarity in the structure. Opal is mainly silica (SiO_{2}) in composition, but it contains in addition an amount of water, thereby differing slightly in refractivity from the original substance. The structure not being quite homogeneous, each crack has the same action upon light as a soap-film and gives rise to precisely similar phenomena: the thinner and more uniform the cracks, the greater the splendour of the chromatic display, the particular tint depending upon the direction in which the stone is viewed. The cracks in certain opals are not filled up, and therefore contain air.” The opal is a very sensitive gem and should not be put near strong acids nor greasy substances. The heat of the body improves its lustre for the opal is essentially a stone to be worn, but it is unsafe to put these gems near liquids or to submit them to fire. “_Truth is as impossible to be soiled by any outward touch as the sunbeam._”