In flame opal , the colour may be seen as red bands or streaks, whereas, In flash opal it appears as sudden brilliant flashes. In girasol the play of colours appears as a floating internal light. Furthermore, the terminology of the opal miner is rather vague, with some words meaning different things to different people. The following definitions may allow some uniformity. There are many types and varieties of precious opal, but western Queensland is renowned for boulder opal.
Boulder opal describes opal found within ironstone concretions of varying shape and size.
The boulder shape is generally elongated or ellipsoidal with the long axis orientated horizontally, and the size ranges up to three metres in length and breadth and one metre in thickness. The direction of elongation usually parallels the bedding in the enclosing sandstone. The boulders may be confined to one or more zones known as boulder levels, or may be erratically distributed through the sandstone. Their composition ranges from sandstone types which consist of a rim or crust of ferruginised sandstone surrounding a sandstone core, and ironstone types which are composed almost totally of iron oxides. Commonly, these concretions have a concentrically banded structure and precious opal may fill concentric, radial, or random cracks, particularly on the underside of the concretion.