Geyser Opal from Spencer Idaho U.S.A Spenser Opal was deposited in volcanic rocks app 4 million years ago by geysers.The primary formation of the Spencer opal mines is a rhyolite and obsidian flow filled with gas pockets. The secondary deposit is a solution of silica and water, trapped within the rock and released by a series of geysers.
Boulder Opal This dark opal is found in Western Queensland and it forms naturally on its host rock of ironstone, which is cut to form part of the gemstone. Boulder Opal occurs as thin veins in sedimentary ironstone boulders. The colour layer is naturally part of the dense brown ironstone base and is inseparable from it.