Rough Opal

Sandstone and boulder opal have been mined in Queensland, Australia since 1872.

Sandstone opal, which is also found in Mintabie and Andamooka, forms opal that is unattached, whereas boulder opal is formed attached to ironstone.

Australian gem associations define boulder opal in this way:
“Boulder opal is presented in one piece, where the opal is naturally attached to the host rock in which it was formed and the host rock is of a different chemical composition composition.”

The earliest miners would cut or break the opal away from the ironstone, and would then sell it as Queensland Opal. If the opal broke when it was being separated, it would simply be thrown away.

It was not until the 1970s that boulder opal found a market. Today, the best pieces of boulder opal can fetch as much high prices as even black opal.