Natural imperfections in opal, explained
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Why natural opal isn't perfect
Opal forms in rock over millions of years, alongside potch (opal with no play-of-color), sand and other minerals. A natural stone usually carries some trace of where it came from. A lab-grown opal doesn't, because it was never in the ground.
Those small marks are part of why no two opals are the same, and part of why we give each of ours a name.
Potch
Potch is opal without play-of-color. It's the same material, but the tiny spheres inside didn't line up in a way that splits light into color. It can be gray, white, amber or black, and it makes up most of what any opal field produces.
Black potch is worth knowing about. It's the natural dark background in a Lightning Ridge black opal, and it's also often used as the backing on doublets.
Lines and patches of potch inside a solid stone are normal, and they can be beautiful.
What's normal
Small patches of potch, faint sand lines and other marks from the ground are common, and they don't affect how a stone wears. Every stone is checked before it's set, and anything that could weaken it isn't used.
Boulder opal is a special case: the ironstone is deliberately left on the back of the stone. It gives boulder opal its earthy look, and it's stated on the certificate as "boulder opal".
A matrix opal ring, a boulder matrix pendant, a Koroit stone with ironstone running through the face: in each case, the dark material is natural rock, not a glued-on backing. That's the difference between boulder opal and a doublet. See boulder opal.
When it's a real problem
Two things are genuine faults: crazing, which is a fine network of cracks across the stone, and cracks that could open up under pressure. Stones with either aren't used in our pieces.
Everything else is character. A natural opal should look like it came from the ground, because it did.
From Opalani
Every certificate states what you're getting: solid natural opal, never a doublet or triplet.