The opals are found in rhyolite geodes (thunder eggs) in a perlite that has altered to pastel colored clay. The geodes that contain gem-quality opal are only about 10% of the total geodes mined and only about 1% of the geodes contain gem-quality opal with play of color. The remaining geodes contain agate, quartz crystals, or common opal. The geodes vary greatly in size, from a few centimetres in diameter to over a meter
