Hi everyone,

I bought a box of gemstones at an auction, and one stone was simply labelled ‘‘Opal Mexico/Peru’’.
I don’t know anything about opals, so I went online and searched for ages and yet none of the images looked like mine.
The rock also fluoresces green under UV light.

Any help would be highly appreciated.

It appears to be potch opal, which is the non-precious opal that normally surrounds and is found with precious opal.

It looks like a type of chalcedony which has many many varieties. There is highly priced chalcedony and not appreciated chalcedony. It is also possible to be potch.

Could potentially be Peruvian blue opal, though not great quality.

both common opal and chalcedony form in similar environments…i.e., wet dry cycles with silica deposition either as hydrated silica or cryptocrystalline silica… in either case, the quality of the stone would not justify the cost of a professional appraisal…