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What is Boulder Opal?

What is Boulder Opal?

Boulder opal is the term used to describe precious opal that has formed as part of ironstone boulders by impregnating the boulder and in-filling cavities and cracks (either partially or fully).

The opal quality may be uniform throughout the vein or may vary with different patterns and colours. Potch bars or intrusions of potch and ironstone will affect quality.

The larger and more uniform patterns of colour are generally found in horizontal veins and pockets. The vertical veins generally produce bar patterns and can also carry gem colour and patterns.

Advantages of Boulder Opal

  • Generally cheaper than a similar looking black opal
  • Larger faces and stones are more readily available
  • Wider range of colours (eg: white, crystal, black)
  • Found only in Queensland, Australia

More on Boulder Opals

Boulder Opal – Uniquely Queensland
  • Australian Boulder opal occurs only in Queensland Australia. Major opal fields in this area include Quilpie and Winton. It has been found as far north Middleton and as far south as hungerford.
  • Australian Boulder opals are the second most valuable type of opal following black opal.
  • Boulder opals are easily distinguished by their layer of solid brown ironstone left on the back of the stone. Boulder opal is found filling cavities and cracks in ironstone concretions called boulders. Due to the general thinness (around 0.5mm to 2mm) of the opal, some of the ironstone is left on the back of the gems forming a stable durable base. Boulder opal is traditionally cut in “free form” shape.
  • Boulder opals, as the name suggests, are mined from large ironstone boulders under the ground. Thin veins of colourful opal forms in cracks and fissures in these boulders. Because these veins of colour are so thin, opal cutters need 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.
  • Our mining efforts have been concentrated around the old Bulgroo opal fields west of Quilpie and in the vacinity of the “Highlands” opal fields East of Yaraka.