Solid opal vs. doublets and triplets
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What they are
A doublet is a thin slice of real opal glued to a dark backing, often ironstone or black glass, to imitate the look of black opal. A triplet adds a clear dome of glass or quartz on top of an even thinner slice. Both are fine products when they're labeled. The problem is when they're sold as solid opal.
From the top, they can be hard to tell apart from solid opal. From the side, look for a straight dark line where the layers are glued, and an opal layer that's perfectly flat.
Why they exist
A lot of Australian opal forms in seams that are only a millimeter or two thick. The color can be beautiful, but the opal is too thin to use on its own. Gluing it to a backing turns it into a wearable stone.
So a well-made doublet is a real product with real opal in it. What matters is that it's labeled as one.
How they compare in price
Doublets and triplets cost much less than solid opal, because there's much less opal in them. That's fair, as long as they're labeled. A very cheap "black opal ring" is almost always one of them. What sets the price of black opal explains why.
Solid opal costs more because it's one whole stone. It can get wet and be worn every day with sensible care. Every Opalani piece is solid, and the certificate says so.
Please don't soak it to find out
People sometimes ask whether they can soak a piece to test it. Please don't. If it's a doublet, you can permanently cloud the glue line. If it's Ethiopian opal, which absorbs water, it can change as it dries.
Look at the side of the stone instead. The full set of checks is in how to tell if an opal is real.