As I mentioned in my previous entry, boulder opal did not have much of a market before 1970. Prior to that time, only opal actually cut out of the boulder was offered for sale.
However, even this boulderless-opal found buyers. According to the book Opals by Fred Ward, these opal first appeared in 1873 at a London gem exhibition, and were so successful that several leases, mines and sales were documented between that year and 1888.
Ward gives Tully Wollaston credit for introducing Queensland sandstone opal to the rest of the world. In 1888, Wollaston set out for Queensland with two other men, to find Joe Bridle, the first miner who found these opals, apparently. Wollaston purchased all he could, then took the material to England to sell. It took some persistence in the face of rejection, but finally some London jewelers purchased the material, and the market expanded rapidly after that, for a number of years – before the other opal fields in Australia made their debut.
Sandstone and boulder opal have been mined in Queensland, Australia since 1872.
Sandstone opal, which is also found in Mintabie and Andamooka, forms opal that is unattached, whereas boulder opal is formed attached to ironstone.
Australian gem associations define boulder opal in this way:
“Boulder opal is presented in one piece, where the opal is naturally attached to the host rock in which it was formed and the host rock is of a different chemical composition composition.”
The earliest miners would cut or break the opal away from the ironstone, and would then sell it as Queensland Opal. If the opal broke when it was being separated, it would simply be thrown away.
It was not until the 1970s that boulder opal found a market. Today, the best pieces of boulder opal can fetch as much high prices as even black opal.
View boulder opal rough available at http://www.opalauctions.com/auctions/rough-opal/boulder-opal-rough/
Boulder opal, which occurs in deposits in weathered sedimentary Cretaceous rocks in the west of the state of Queensland, is unique. For one thing – it is found only in Queensland!
Boulder opals are the second most valuable type of opal (after black opal).
Boulder opals are easily distinguished, because they have a layer of solid brown ironstone on the back of the stone. Boulder opals are mined from large underground ironstone boulders. Thin veins of colourful opal forms in cracks and fissures in these boulders. Because these veins of colour are so thin, it is necessary to leave the ironstone on the back of the opal to form a full sized stone. The thin layer of opal in boulder opals can display any colour of the spectrum in a beautiful play of colour.
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—these are called ‘Yowah-nuts’.
The major opal mines are:
–Winton
–Quilpie
–Yowah
–Koroit
–Jundah
Boulder opal rough needs experienced cutters to work the veins. The veins are tricky to follow. Many cutters use saws to cut and split the boulder rough — the rainbow colours can be spectacular.
When a miner has cut a piece of boulder rough and sees a good vein of colour, he will put a nick along this vein colour with his saw blade and the rock will split open revealing the opal colours. This is termed “a split.”
