Boulder Opal Boulder opal is the second most prized form of opal, after black opal. The name derives from the fact that boulder opal is found embedded in ironstone boulders. The opal usually forms as thin veins within these boulders, and most stones are cut to include some of the host ironstone matrix. Boulder opal is sometimes referred to as opal in matrix for this reason. Boulder opal is especially attractive because, like the black opal, it has a dark body tone which adds vibrancy to the play of color. Boulder opal also has a higher density because of the ironstone content, and can be more durable as well.