What is Boulder Opal?

Queensland Boulder Opal is one of the world’s finest and most exotic gems with many forms and colours. Each piece is as unique and individual as the person who chooses to possess these exquisite gems.

There are quite a few forms of Boulder Opal, these on their own have unique and interesting sub-varieties which have brand names of their own.

Like stunning “Full Face” boulder which is generally a full clean face of Opal with a natural backing of the ironstone(boulder) left on the back, to help support the veneer of natural opal formed on the face.

Sometimes Boulder Opal can provide beautiful pairs when the seam is cleaved to expose a two sides of opal from the one seam.

All forms of Boulder opals can either have a cabochon or an undulating surface, dependent of the thickness of the opal, which are generally cut in a free-form shape for this reason, allowing the opal cutter the freedom to combine their creative skills with the deposition of the opal.

Boulder Opal can sometimes be formed in a cavity of a mould left behind from a decomposing tree or plant matter that has pseudo-morphed  into ironstone , if these then infill with opal you can get rare and striking wood replacement gemstones , with the striation of the wood pattern with opal which are awesome and look similar to a cathedral window!

Koroit Opal

Koroit opal a unique masterpiece of creation, which can be full face or matrix or a combination of them both Matrix is when the opal permeates smaller cavities within the ironstone host in ultra thin seams or nodules.

The Koroit opal field is known for the very distinctive type of boulder opal that is found in the mines in this area. 

Koroit Opal is widely known for its unique character, variety and intensive colour play; in particular for its pattern opal, picture stones, centre nuts, matrix, opalised wood and beautiful bright clean face opal.

Due to its originality and quality, Koroit opal is highly sought after opal lovers, wholesalers and retailers around the world.

However travellers beware - unlike Yowah, the Koroit Opal field is not a tourist destination, these are active mining fields and not for tourists, there is no fossicking or services available on these fields unless you know a miner there.

The field consists of some clusters of mostly commercial opal claims and leases, situated several kilometers away from each other. 

For the last 25 years Koroit opal production has been at its most substantial since the fields discovery.

Koroit Opal Field History

The Koroit Opal field was discovered in 1897 by Lawrence Rostron, the manager of Tillboroo Sation, Eulo.

Mining continued for a number of years mostly by syndicates until by 1905, digging approximately 150 shafts up to 15 metres deep and 472 metres of drives, finding 300 ounces of medium and 30 ounces of extra fine gem grade opal with another one and a quarter tons of gem grade matrix.

Koroit Opal field become dormant shortly afterwards, competing with an oversupply of opal from Whitecliffs (NSW). Some mining resumed in 1927 until e outbreak of the second World War, where Australian Diggers were once again called to arms in service to their nation.

More recent mining events started again in 1972 when the popularity and profitability of the gemstone increased.

Mining in Koroit today done by either open cut large scale mining or some still prefer to underground mine, albeit a little easier utilising electric lighting, jack hammers,diggers boggers and mechanical hoists.

Koroit Opal today competes with no gem, it is a stand alone brand of its own with the Koroit field and its unique characters supplying magnificent opal to opal connoisseurs around the globe.

Yowah Nuts

Yowah Nuts also have their own unique form, with the ideal form being an Ironstone concretion (which can vary in size) which hosts opal filled void in the centre of the Yowah nut.

Yowah nuts also occur with a combination of opal in a matrix skin which infills voids within the outer ironstone layers and also can have an opalised centre filling as well.

The Yowah nut is generally sawn down the middle to reveal its hidden opal center, which provides lovely matching pairs.

The Formation of Boulder Opal

Boulder opals, as the name suggests, are mined from large ironstone boulders which are formed from “diagenetic and ionic bonding” process, from iron and /or manganese rich sandstones, generally in ancient paleo-channels, lake beds or streams. These boulders can form into small or massive sizes and once dried in situ amongst the sandstones, form cracks or cavities as moisture is dehydrated.

Opal can then forms at any later stage in these cavities in these boulders if silica rich solution passes in and around the boulder. 

There is still some contention about the source of this silica with “old school” theories pertaining to “ancient inland seas” being the source, or the newer theories about hydrothermal venting from the artesian basin, travelling upward through major faulting to surface or subsurface “blows” which re-deposits silica which has been dissolved through high temperature during its travels through the underlying strata and redeposited in boulders if they are situated within the right parameters near the blow.

Future scientific examination needs to focus on opal formation and the age of opal to better understand how Opal was formed, which will unlock the secrets and enhance future exploration for this unique and wonderful gem!

Where is Boulder Opal found?

Boulder opal is unique to Queensland and is found stretching from Koroit opal field, in the south west to Kynuna in the North West of the State, and includes within the well known opal fields of Yowah, Eromanga, Quilpie, Jundah,  Opalton and Blackall to name a few.

Queensland's opal endowment is the largest opal deposit in the World.

This Queensland Boulder opal deposition is spread out over a vast region approximately 1200 kilometres long and 400 kilometers wide and occurs with the Winton Formation, which is a geological series of the Cretaceous period which occurred between 145 -66 million years ago.

Quilpie Shire is "Home of the Boulder Opal",  where Boulder Opal was first discovered on Listowel Downs,  a property in the Quilpie Shire in 1869.

Boulder Opal has been mined in the State since early European settlement, and has been a mainstay of employment opportunities in Regional Queensland , particularly when recessions and depressions limited ones capacity for employment with many turning their hand to working opal in the Western Queensland Opal Fields through these times of economic hardship.

Many returning soldiers, returned to the opal fields for respite and solace, as they and  their gold mining mates tunneling skills, attained in peace, were utilized by their military commanders during both World Wars and are immortalized in Australian Culture by the term "digger" a badge of honour that is inherited and proudly embraced by past and present Australian Soldiers earned by the daring and courageous men in who's tracks we now walk.

Whilst many of the mines discovered by these diggers have given way to modern mining, which has removed the  evidence of their old workings, they can still be discovered and are a testament to their skills and fortitude, particularly when you consider the technology and transportation and isolation of the era.

Famous Opal Mines in the Quilpie Mining district include, “The Hayricks”, Bull’s Creek , The Hammond’s and "Little Wonder", to name a few, all which which have supplied the International Jewellery Markets with some of the finest of Queensland Boulder Opals , delighting enthusiastic customers with the unique colours and forms that Boulder opal can offer.