What is Boulder Opal?
Boulder opal is the term used to describe precious opal that has formed as part of ironstone boulders by impregnating the boulder and in-filling cavities and cracks (either partially or fully).
The opal quality may be uniform throughout the vein or may vary with different patterns and colours. Potch bars or intrusions of potch and ironstone will affect quality.
The larger and more uniform patterns of colour are generally found in horizontal veins and pockets. The vertical veins generally produce bar patterns and can also carry gem colour and patterns.