Solid brown ironstone boulders have thin veins of opals that form in cracks and fissures. Boulder of Opal is a dark-siliceous ironstone matrix – a mineraloid with an internal structure. The veins of boulder opal are so thin that the host rock, ironstone, remains on the backside to form a full sized stone. The formation is completely natural. Boulder opals are almost always cut in freeform shape.
* Scientific proof may not be available to substantiate these claims.