The word "phony" is said to come from fáinne, the Irish for "ring", based on the idea that Irish emigrants ran the found-ring scam in the US in the 19th century.
Someone tried it on me in Paris; I said "Oh, lucky you - when you bring it to the police, they might give you a reward. Oh, look, there's a policeman - Hi!" - and as I waved to the imaginary flic, the finder took off.
Some hours later I passed the same person trying it on an elderly American tourist, who was gazing at the "gold" ring with glasses perched down his nose. I said as I passed, "This is a thief", and got the filthiest look ever from the phony.