Australia's main types of natural precious opal include:

• black opal from Lightning Ridge in New South Wales
• white opal from South Australia
• boulder opal from Queensland
• matrix opal

Small amounts of precious opal of volcanic origin also occur in basaltic lavas and pyroclastics.

Boulder opal

Boulder opal is unique to Queensland. It occurs in deposits in weathered sedimentary Cretaceous rocks in the west of the state.

It is found in siliceous ironstone concretions or boulders ranging in size from less than a few centimetres to more than 20 cm.

Concretions up to 5 cm across, known as 'nuts', may host a kernel of solid opal or contain a network of thin veins of opal through the ironstone. This variety of opal is prevalent at Yowah where the concretions form distinct bands-the well known 'Yowah-nuts'.

Matrix opal

Only a small proportion of boulders contain precious opal. Others contain matrix opal. In these,the opal occurs as an infilling of pores or holes or between grains of the host rock (ironstone). Varieties of boulder opals are defined by body tone, play-of-colour and transparency. They include black, dark and light variations.

Occurrences

Queensland's opal fields are the west and south-west of the state. They include:

• Yowah field (the southernmost field centred on the small town of Yowah-includes Black Gate)
• Koroit field (north-east of Yowah)
• Toompine field (east and south-east of Toompine-includes Lushingtons, Coparella, Duck Creek, Sheep Station Creek and Emu Creek)
• Quilpie field (west and north north-west of Quilpie-includes some of the more productive mines in recent times-Pinkilla, Bull Creek, Harlequin, and probably the most famous of all, the Hayricks.
• Kyabra-Eromanga field (west and north-west of Eromanga)
• Bulgroo field (north of Quilpie field in the Cheviot Range-includes the Bulgroo, or Germans-and Budgerigar to the north )
• Yaraka field-includes the mines in the Macedon Range, such as Mount Tighe
• Jundah field (west of Jundah over the Thompson River-includes Jundah and Opalville mines)
• Opalton-Mayneside field (centred on the old abandoned township of Opalton, and to the south in the Horse Creek - Mount Vergemont area)
• Kynuna field-the most northerly field, south of the township of Kynuna.

These opal fields lie within a 300 km-wide belt of deeply weathered Cretaceous sedimentary rocks known as the Winton Formation. This extends north-north-west from Hungerford on the New South Wales border, west of the townships of Cunnamulla, Quilpie, Longreach and Winton, to Kynuna, a distance of about 1000 km.

Boulder opal is widely distributed in rocks in these areas, in generally elongated or ellipsoidal ironstone concretions or boulders, from a few centimetres, to up to 3 m across.

The boulders may be confined to one or more layers or randomly distributed through the weathered sandstone. Their composition ranges from sandstone types (a rim or crust of ferruginised sandstone surrounding a sandstone core) or ironstone types (composed almost entirely of iron oxides).

The opal occurs as a filling or lining between the concentric layers or in radial or random cracks in the ironstone, or as a kernel in smaller concretions or nuts. (as found at Yowah and Koroit fields, the famous 'Yowah-nuts').

Matrix opal is where the opal occurs as a network of veins or infilling of voids or between grains of the host rock (ferruginous sandstone or ironstone).

Rare seam or band opal is also found and is typically encased in ironstone.

Pipe opal occurs in pipe-like structures which may be up to several centimetres in diameter within the sandstone and these structures may be hollow or opal-filled.

Wood opal is occasionally found replacing woody tissue material.

As opposed to other sedimentary precious opal, boulder opal is attached to the ironstone, and stones are usually cut with the natural ironstone backing intact. Solid opals may be cut from the ironstone material where the opal is of sufficient thickness.

Boulder opals are fashioned to standard shapes and sizes but are also cut in freeform shapes to highlight their individual beauty and to avoid wastage. Magnificent picture stones are also cut but these are mainly of interest to collectors rather than for jewellery use.

Mining, production and outlook

The larger mines operated under mining leases in recent years are large open-cut operations.

Overburden is stripped from zones of ironstone boulder concretions. Boulders are carefully removed from the ground for processing. Heavy equipment has been used to open up most areas of old workings.

Underground methods are still applied with success in some areas. Shafts are sunk until a prospective layer is intersected, rapid sinking of shafts being accomplished with Calweld bucket drilling rigs. Miners use 'light' electrical machinery driven by portable generators.

If the boulders show any evidence of opalisation, they are first removed from the mined ground and collected for later inspection for opal content, and sorted in readiness for sale as rough, or for further processing.

Opal is cut and polished into cabochons or free-form shapes. These may be solid stones, doublets with a dark coloured backing, or triplets where a thin slice of opal has a quartz or glass capping fixed to the top as well.

For boulder opal, some of the ironstone is left attached as a natural backing, producing natural doublets. Other stones are cut from the ironstone matrix containing opal.

The Australian export market for opals has been in decline in recent years, but this trend is likely to be turned around as production increases from existing and new deposits. Australia exports most of its opal to Hong Kong, Taiwan, Japan, USA and Europe.

As most old diggings have now been largely worked out or reworked by open-cut methods, exploration to find new prospects has commenced over known opal bearing country.

Well-resourced exploration companies have become involved and are applying more systematic and extensive exploration techniques on a regional scale in the search for new deposits. Positive results have been reported, which should lead to further exploration and new mines.

In addition to the traditional opal fields, exploration for opal has been undertaken in the Hebel-Dirranbandi area near the Queensland - New South Wales border where there is a 70 km northern extension of the Cretaceous Griman Creek Formation, which hosts the Lightning Ridge opal field in New South Wales.

There are opportunities for further exploration and mining of opal in Queensland by companies as well as individual prospectors and miners.