Williams Opal Mine (Three Mile Hill Mine), Coolgardie, Coolgardie Shire, Western Australia, Australiai

Regional Level Types
Williams Opal Mine (Three Mile Hill Mine)	Mine
Coolgardie	City
Coolgardie Shire	Shire
Western Australia	State
Australia	Country

Australia produces about 90% of the world's precious opal. However, in Western Australia, it is a very rare species. It has been found in only two locations in an area amounting to the size of western Europe.

Williams opal mine is located 4.8 kilometres north north-east of Coolgardie.

Precious black opal was mined at Williams Opal Mine in 1904, but abandoned shortly after due to the limited amount of opal available. All fell silent until the 1960's when an open-cut was developed over the old workings. Only a few blob-like concretions containing precious opal were recovered. The mine was worked again in 1989, extending the pit to 10-12 metres deep.

Opal is found in a 9 metre wide shear zone, within graphitic slate bands grading to thin feldspathic bands. The bands have silicified, and at one locality thin seams of precious opal have developed along cleavage and joint planes. The siliceous matrix is a relatively hard, black or dark grey, fine grained rock, and is jointed and foliated. A series of quartz veins have been intruded by this rock, and both are cut by a series of minute cracks up to 2.5mm thick filled with opal from precious to parts of no commercial value.

Most of the opal is in thin seams, and slivers were unfortunately mounted as jewellery rather than being retained as mineral specimens. The opal was black and grey, displaying red, orange, green and blue play of colours. The surrounding potch veins were wider and were glassy and bluish in colour. Mineral specimens from the location are very rare.

In 1989 the lease containing the Three Mile Hill opal mine was purchased by Peter Milic, an experienced opal prospector and miner who had previously worked at opal fields in South Australia, Lightning Ridge (NSW) and western Queensland. He began drilling and blasting at Three Mile Hill and immediately found small fragments of precious opal. According to Wattone (1992), at the end of September 1991 Milic recovered a magnificent piece of black opal weighing 16.5 kg, by following up small traces of opal in one area of the pit. Two days later another find of 2.5 kg and a third of 6 ounces was made. The large specimen was valued at AUD$5-6 million when appraised by valuer in Perth but no immediate buyer was found. The large piece was cut and taken to Adelaide, presumably to be sold. Some pieces from the 1991 find were apparently valued at AUD$5000-10000 per carat.

Other Regions, Features and Areas containing this locality

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