What is an Australian boulder opal "split " ?

An Australian boulder opal split is when an opal vein in a boulder opal has been split into 2 pieces.
Australian boulder opal is precious opal found in cracks or cavities in an ironstone rock.
When an opal vein in a boulder is thick enough, a groove of about one millimeter deep is done around the stone into the opal seam with a diamond saw blade.
The stone is then split open. It results in 2 stones instead of one, almost identical, fitting together.
Depending on the size of the original stone, a pair of earrings, or a full set of earrings, pendant and a ring can be obtained with matching opal colours.