Boulder Opals are natural formations of thin opal on ironstone. The opal appears naturally in thin seams between the ironstone rock, and is cut in such a way as to retain the ironstone backing.
Boulder Opal is very strong because of the naturally melded ironstone, and the dark backing gives the opals the same benefit as a dark backing on a doublet opal, however the value of boulder opals is higher since this occurs naturally.
Queensland Boulder Opal is know to display fantastic colours similar to black opals. They can be cut into oval pieces, but are more often cut into irregular shapes that make for fantastic pendants.
You should be weary of buying boulder opals on a per carat basis as the ironstone is heavy and has very little value, however boulder opals are far more valuable than doublet opals and triplet opals..
Lapidists (gem cutters) need to be especially careful when working with boulder opals as the ironstone is much harder than the opal, and as such, the opal wears away more quickly during the polishing process.