Notes:

Boulder Opal - Boulder opal also has a dark body tone (although there are some occurrences of white boulder opal), which is as dark as black opal. Because boulder opal forms in thin veins in ironstone boulders, the host ironstone is usually left on the back of the stone. This is why, if you look at the back of a boulder opal, there is a layer of brown rock attached to the back. For this reason, boulder opal is sometimes referred to as a 'natural doublet', as this layer of dark stone on the back gives the opal its dark body tone.