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Boulder opal is opal that forms inside an ironstone concretion. The concretion was formed was formed due to ionisation, from sedimentary deposition. By definition, they are ionised concretions of varying hardness with an approximate opal composition of SiO2 at 28%, Fe203 + AL203 at 68% and H2O at 1% composition.
The opal forms within the cavities of the concretion in both vertical and horizontal cracks. Boulders vary in shape and size, from elongated to ellipsoidal in shape and from as small as a pea too as big as a family car.
Boulder opal has a tendency to cleave, when cleaved the 'split' as it is often referred leaves the beholder with two faces of opal, with a naturally polished face. This phenomenon is due to the angle the microstructure of the opal originally grew.
The opal cutter commonly finishes the opal, with the natural opal veneer exposed with ironstone backing. Although Queensland produces other forms of opals, sandstone opals as pipe and seam and matrix, which are often found at the same mine, it is the Boulder opal that is the most sought.
Extracts from a Journey with Colour
Published by Len Cram
The history of opal in Queensland is one of heartbreak, frustration and at times success at incredible odds. Rich in myths and legends, Queensland is the birthplace of the Australian Opal Industry.
Opal was first discovered in Queensland on Listowel Downs, south of Blackall in 1869. The first registered mine was in 1871 south of the present town of Quilpie.
Among the early miners were Berkelman and Lambert, who worked a deposition on the Barcoo in 1872-1873, and whose opal attracted great interest at the Queensland Annexe of the London International Gem Exhibition in 1873.
By 1875 there had been a couple of wonderful finds and interest began to grow, but it wasn't until 1888 that Tullie Wollaston, a young surveyor turned entrepreneur from Adelaide made a determined effort to market the gem that we now have a viable industry.
Opal gougers of last century were mostly shearers and station-hands who had little or no geological knowledge. George Cragg, a young stockman, discovered the northern opal fields on Warronbool Downs 100 kilometres south of Winton where the Opalton fields exists even to this day.
Two World Wars and droughts slowed the progress of Boulder Opal realising its full potential on the world stage. Although mining on a small scale continued it was relatively dormant. It was not until 1967, when Des Burton, a pharmacist from Quilpie became involved with boulder opal, unwittingly through his efforts, helped revitalise an industry. In the 1970's he introduced modern opal cut mining techniques which revolutionised the opal mining industry.
Boulder Opal and the people that mine and deal with opal have supplied the industry a rich and colourful history, which has become part of Australia's heritage. Opal has been discovered in Queensland from the Southern Borders of Western Queensland to as far north as Kynuna, this would probably be the largest opal field ever known, with opal mining centres in Winton and Quilpie.
Today the opal miner still exists, supplying the markets of the world with this most exquisite product, Queensland Boulder Opal.
Opal mining has changed significantly since the first opal lease 'Pride of the Hills' was pegged in 1871. The opal miner of that era would generally utilise what he could carry, if he were lucky he would have livestock to assist in the task. Generally a pick and shovel were his tools; a rope bucket and a hand fashioned windlass made from local timbers were fashioned on site.
It was generally not the work or the heat that would foil the attempts of the miner to win the opal but the need for water, with searing summers reaching 52 degrees centigrade the Australian Outback can be hostile. However, this summertime weather bought the rains to the Outback and miners would often camp out and mine until these water holes dried up.
Today underground mining can still be seen, however the use of motorised windlass, pneumatic or electric hammers make the task easier than it was in the humble beginnings.
Mechanisation introduced to the opal miners a new way of extracting the earth's riches, what a hand miner would dig in a week, a bulldozer could do in an hour. Today excavators, bulldozers, front end loaders and dump-trucks are a common sight on the opal fields.
Opal mining today is being conducted more professionally, with more stress on the miner to conduct his business in a viable manner with Environmental legislation to conform to. Today the miner has as his disposal technical support via the Internet that provides up to date legislation, mining tenure information and geological information of opal occurrences.
The cutting of opal is currently a skill acquired by hands on experience or taught by lapidary clubs.
Boulder Opal is significantly more difficult to work, as the nature of the material forms to the cavity in the ironstone, the seams of opal that run through the boulder can be of very excellent quality but are very thin veins from .25mm to 20mm thick. The ironstone are generally left as backing to support the stone.
Occasionally a thick vein is deposited allowing the cutter to cut the opal in cabochon, however frequently these thin veins are wavy, the cutter is than challenged to cut and polish the piece following the deposition of the opal, resulting in an undulating or baroque surface.
Stones are generally cut into freedom shapes, which are dictated by the colour deposition and flaws within the piece. The end results of the cutter's skills provides the beholder a unique piece. No two opals are exactly the same, which insures the owner of individuality when selecting this wonderful gem.
The marketing of Boulder Opal depends where in the chain the producer or buyer operates.
In general the miner sells their rough product to cutters who process the material and resale the cut and polished pieces to wholesalers who in turn remarket the product to retailers or export to world markets. Although this may be the norm, today more and more miners are processing their opal and selling direct to anywhere in the chain so as to maximise their profits.
Generally it is the case, that purchasing direct from the miner is the most economical way for a buyer to acquire product. Local and overseas buyers frequently visit the opal mining fields in both Winton and Quilpie to maximise their buying power and hence their profits upon resale.
Extracts from 'Opal the Gem of the Never Never'
By T.C Wollaston in 1924
Australia is the home of the opal. It is only what one might have guessed, for it is just that kind of home, with the ample room and happy conditions, which enables such a radiant child to grow and express itself in so many different ways.
The Australian Opal is the child of the desert and delights in the Never Never, for in it's babyhood it paddled there in the shallow inland, which covered our vast interior like a silver quilt, romped with the periwinkles and mussels and fan shells, teased the sick lizards and captured their knucklebones when they died and took possession of the little harsh sponges and the grey fluted corals and slipped into every odd corner or snug little holes where the water were drying up and where the sun could be sipped at leisure.
It grew grain by grain to the music of the dripping water, true and unhurried, building up a glad-eyed responsive nature like a baby does. And just as any active babe when it sleep is over kicks off its clothes in its love for freedom, any vigorous opal feeling the cleat warmth through the thin covering of earth, could in its brisk impatience kick off the quilt, and bask on the burning ridges where one may catch the gleam of its starry eyes now and then as one rides through those grey solitudes.
Whereever the Greeks and Romans obtained their opals they were evidently rare. Marc Anthony, it will be remembered, coveted so much the opal ring of Nonius, to give to Cleopatra his sweetheart, that he banished Nonius for refusing to part with his treasure.
All down the ages opal has stirred the artistic souls and passionate hearts of mankind, the Romans as well as the Greeks being lovers of "this miracle and Queen of Gems" as Shakespeare hath it in "Twelfth Night". It was not only known centuries before Christ, but prized above all other gems, and became the symbol of hope and purity.
