Black opal pairs direct from opal mines in Australia. It is considered rare just to find a black opal these days, so to find a pair matching is a rarer still. Each opal is so unique at lightning Ridge to find a matching pair for opal earrings can be very difficult with black opals as there is never large selection available.
Opal pairs are formed in two ways
First opal is formed as knobby this is generally the darkest body tone and more expensive type of opal pairs. The opal cutter would slice the knobby which is shaped like normal rock in two parts. But the nobby might not have formed even so it is common for one section to be slightly larger or higher or even a bit brighter than the other half and one part might have potch or a sand dot so it is really hard to get exact matching black opals compared to other gemstones where a pair can be exact.
The other option for opal pairs is seam opal. This opal is formed in layers more so than just single piece rock, so opal miners will have larger flat piece and can cut matching earrings. These are generally semi black or dark tones to crystal sets
Crystal or white opals from coober pedy do make matching earrings but ti is the uniqueness of black opals that attract buyers.
Some opal pairs for earrings are cut from the same rock so they are ideal for a matching ring or pendant or for a couple to have one piece each like ying yang.