Rough black opal (opal rough black) is often sold in the form of rubs

Opal Rough black comes mainly from the Lightning Ridge area but has also been found in Andamooka, Mintabee and Coober Pedy, Sydaustralien

This means that the areas containing color are cleaned out of the valueless potch or ironstone, leaving only the valuable part of the stone. This saves time and money for the opal cutter