Rock & Mineral Encyclopedia

Search and identify 1,000+ rocks, minerals, crystals, and gemstones — with properties, formation, colors, hardness, and how to tell them apart.

Boulder Opal

Precious opal that forms in thin veins within brown ironstone boulders, cut with the host rock left as a natural dark backing.

Boulder opal from the Quilpie district of Queensland, Australia, with bright color set in dark ironstone matrix.

Boulder opal from the Koroit field in Queensland, famous for intricate ironstone matrix patterns laced with colorful precious opal.

Opal from Madagascar spanning colorful common opal and some precious opal, including pink, green and boulder-type material.

Opal in which precious play-of-color is intimately dispersed through the pores of its host rock rather than forming a solid seam.

Opal from Australia, the world's leading source of precious opal, ranging from white and crystal to prized black and boulder types.

Small ironstone concretions from Yowah, Queensland, whose hollow or veined centers hold brilliant precious boulder opal.