THE AUSTRALIAN MUSEUM MAGAZINE. 



201 



rock known as andesite, and is usually 

 in small stones, though occasionally 

 larger pieces have been found, and one 

 preserved in the Vienna Museum is as 

 large as a man's fist and has been valued 

 at £3,500. 



WHITE CLIFFS. 



Australian opals have now almost 

 ousted the European product from the 

 world's markets. Precious opal was 

 found filling cavities in the decomposed 

 basalt of the Abercrombie Ranges, New 

 South Wales, in the 'seventies, but 

 Australian opal did not become impor- 



a sulphate of soda and lime, forming 

 aggregates which from their appearance 

 have been called fossil pineapples. 



Much of the opal found at White 

 Cliffs has very little or no value for gem 

 ]iurposes, and is known as ])otch, but 

 the field has ]iroduced many fine stones. 

 In valuing opal, several ])oints must be 

 taken into account. Colour is the most 

 im]>ortant, red fire, or red in combina- 

 tion with blue, green, and yellow, being 

 considered the best. Pattern is also an 

 important factor. In pinfire opal the 

 colour is in very small ])atches, almost 

 pin points. Harlequin 0])al has the colour 

 distributed in small, fairly regular 



Hm^- 



,j»^^^' 



A- 







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-«►. 



A Lightnins^ Ridge Opal Mine, showing the mouth of the shaft and 

 the windlass by which the broken rock is hauled to the surface. 



[Photo.— y;r. J. V. Vaiiex 



tant until the White Cliffs field was 

 o]>ened up. In 1889, a hunter was track- 

 ing a wounded kangaroo in the drought 

 stricken region beyond Wilcannia, and 

 noticed a brilliantly coloured stone, 

 which he picked up and retained as a 

 curio. Other discoveries followed and 

 soon the White Cliffs opal mines were 

 in full swing. 



At White Cliffs the opal occurs in a 

 white siliceous rock, a kind of sand- 

 stone, of Cretaceous age, filling cracks 

 and seams in the rock, or replacing the 

 material of wood, shells, and reptilian 

 bones. It also occurs as a replacement 

 of a spikey mineral, probably glauberite. 



squares. In flash opal the fire shows 

 as a single flash in large patches. 



BLACK OPAL OF LICxHTNING RIDGE. 



The most valuable opal now comes- 

 from Lightning Ridge, near Walgett, 

 New South Wales. This field was dis- 

 covered about 1895, and has produced 

 some of the finest opal ever found. The 

 opal occurs here in a sandstone con- 

 taining much iron, and the iron is 

 doubtless responsible for the dark colour 

 of the matrix, which makes a splendid 

 setting for the wonderful colours shown 

 by a Lightning Ridge black opal of 

 good quality. 



