Solid, doublet, triplet
Three types of opal jewelry stone. The one question every buyer should ask before handing over money.
A solid opal is a single piece of natural opal, cut and finished on its own. No assembly. What you see is the stone. Solid is the standard for gem-quality material, and when no qualifier is given, it's what you should assume you're buying — until you check.
A doublet is a thin slice of natural opal glued to a dark backing, usually ironstone or black industrial glass. The backing provides the dark ground that makes play-of-color pop; the opal layer provides the color. Doublets aren't fraudulent — they're a legitimate way to get a usable stone out of material too thin to stand alone — but the price should reflect that you're buying a manufactured assembly, not a solid stone. How to tell: look at the girdle edge. A doublet has a visible join line between the opal layer and the backing.
Triplets
A triplet adds a clear cap — usually quartz or glass — on top of the doublet, doming the surface and protecting the thin opal layer. Triplets are inexpensive, can look vivid, and are often sold in tourist markets and mass retail. The opal layer is typically extremely thin, the cap distorts the color slightly, and the glue joins are vulnerable to water infiltration over time. Solid, doublet, and triplet all get listed as 'opal jewelry.' The question to ask any seller is which type. If they don't know, that's an answer too.