Australian boulder opals are prized for their natural rarity and unique formation that gives them a striking visual appeal. Unlike opals mined from loose deposits, these gems form within natural ironstone boulders and retain a piece of that host rock, lending each stone an unmistakable, organic character. The dark, weathered ironstone provides a dramatic, earthy backdrop to the opal’s brilliant flashes of color, which can range from deep, fiery reds and vivid blues to subtle hints of green.