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Geosciences LibreTexts

16.50.03: Boulder Opal

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    This unique opal is spread over thousands of miles and known as the Winton Formation, which is millions of years old. Famous Boulder opal fields include Winton, Quilpie, Opal town, Jundah, Bulgarro, and Kyabra.

    Koroit and Yowah are two opal fields close together and are now producing popular opal that has more pattern and unique style more than color as most boulder opal fields.

    We have now added these fields under Boulder opal as many new buyers miss these categories. They have a reputation for veins of color in spectacular patterns compared to other Boulder opals fields

    Koroit and Yowah generally have more dark chocolate ironstone backing and other opals fields have more sandstone light brown color and both can display veins caramel color.

    Boulder opals have more variety and uniqueness compared to other opals and gemstones as so many factors have taken place over million years and different mineralization has helped in the huge cross spectrum of unique boulder opals that you can enjoy

    Another interesting type of Boulder Opal is the wood replacement fossil. These stones have replaced a cavity left over by old wood. This creates a stone with a unique wood pattern. Check them out, they are amazing.

    Please note some color bars on this opal is very thin and sometimes buyers mistake for doublet which is man-made by joining crystal opal to ironstone.

    To check that it is genuine opal, best to have a magnifying glass and view color bar and you can see it isn’t glued by following color veins. Also, you may place it in the freezer for a few minutes and if it's a doublet, the glue would crystallize and become easy to see.

    It is an extremely stable type of opal that does not generally crack or craze. Since it is mixed with ironstone it is very strong and resistant to breaking, but some boulder opals do show crack lines and most of these are caused when the opal was formed as many times you will split open a boulder opal to see crack lines which could have been caused by shifts in the earth to earthquakes or movements when the opal was forming.

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