Boulder opal is found in Queensland from Kynuna in the northwest to Hungerford in the south. The deposits are scattered and remote with some mines being almost 100km away from the next.
Boulder opal consists of a layer of opal naturally adhering to an ironstone, mudstone or sandstone backing. The opal is found as thin to thick veins running through the ironstone concretions. These are found generally in sandstone or clay levels within the opal country.
Boulder opal is cut leaving some of the natural ironstone backing under the opal. Ironstone intrusions through the surface are not uncommon and may or may not detract from the value of the opal.
A “clean faced” boulder opal with little or no ironstone or sand showing in the face of the gem is the most sought after form of boulder opal.
The cost of boulder opal is generally much less than a similar looking solid black opal from Lightning Ridge. It is not uncommon for a boulder opal to exhibit much brighter colours than most black opal.