Koroit Boulder Opal

Koroit boulder opal is found in Paroo Shire in southwest Queensland, Australia. The opal is found in a sandstone matrix called the Winton Formation, which contains ironstone concretions. The opal forms within the layers in the sandstone and as kernals in the concretions. The brown and red banding of the sandstone combined with the multi-colored precious opal make this a stunning material for cabochons. Boulder opal with its combination of sandstone, ironstone and opal, has different hardnesses throughout the stone. Because of this, the material may be stabilized or backed. I have made boulder opal cabochons without stabilization, but you must use care when shaping as the ironstone is much harder than the sandstone or the opal.