Several different websites I’ve checked explain the differences between natural, synthetic, and imitation opal. Synthetic/lab-grown was mostly used to refer to man-made opals that used hydrated silica to recreate stones that were chemically and structurally as close to natural ones as possible, meaning hydrated silica. While imitation opal was described as anything that even vaguely looks opalescent, including glass and plastic, which is probably the source of a lot of scams.

It also sounds like the topic of lab-grown opal is just as controversial as synthetic diamonds. With purists insisting that natural is better despite steady improvements to the methods and technologies leading to synthetics that are increasingly difficult to differentiate from natural.

At least that’s what I gathered. I’m not an expert, didn’t even finish my geology course in college. Lol. Correct me if I’m wrong.