the minerals of new south wales. 197 



Opal. 



This mineral consists of silica, with usually from 5 to 12 per cent. 

 in water. 



Precious or Noble Opal: The precious opal of New South 

 Wales has the milky body colour usually possessed by this mineral, 

 and the same brilliant play of colours ; the dominant colours of 

 the scintillations are metallic green, pink, and red. Some of the 

 best specimens form, when polished, very fine gem-stones ; but 

 here as elsewhere the valuable specimens obtained bear but a 

 small proportion to the whole. The best have been obtained 

 from Rocky Bridge Creek, Abercrombie River ; the matrix is a 

 fine-grained bluish-grey amygdaloidal trachyte, which is so much 

 altered that it can be abraded by the thumb-nail ; the opal has 

 filled by infiltration certain of the vesicular cavities and crevices 

 in this rock ; it is associated with much common opal free from 

 any play of colour. 



The appearance and mode of occurrence of the opal found at 

 Bulla Creek, in Queensland, is very different ; the body colour of 

 the Queensland opal is usually deep ultramarine blue or green, 

 and the reflections are usually metallic green and red • the matrix 

 is in this case a brown mottled clay porphyry, in which the opal 

 occurs as small veins and strings. 



Opal is also found in a similar clay porphyry in the "Wellington 

 District ; but up to the present I have only seen very small 

 particles of the precious opal diffused through much valueless opal ; 

 also occurs at Bland, near Forbes ; also at Coroo, with chalcedony, 

 agates, &c. ; and at Bloomfield, near Orange. 



Fire Opal or Girasol—(i.e.) An opal with a red or orange tint — 

 occurs at Wellington. But little valued. 



Common Opal, Semi-Opal, and Wood Opal : Are common in all 

 the basaltic districts ; Uralla, Inverell, Richmond River, Trunkey, 

 Scone ; Hunter, and Castlereagh Rivers, Kiama, Lachlan River. 



Cacholong : Dead white ; conchoidal fracture ; adheres to tongue. 

 Tumut. 



Hyalite : Muller's Glass. 



Found coating the joints in basalt, Jordan's Hill, Cudgegong ; 

 of a blue colour at Ororal. 



Silicified Wood. — Is very abundant over nearly all the basaltic 

 districts ; the two have always been observed in close proximity. 

 It is very abundant also throughout the coal measures. Large 

 boulders of such fossilized wood are met with in most of the 

 drifts and river deposits. 



