The Britannia coin, I'm fairly sure is a modern replica. Being the first coin ever to depict and name "Britannia", it's a very popular coin in Britain - and so, one that is very often replicated and sold at ancient tourist-themed locations.
As for the squarish-shaped thing, I'm fairly certain it's not gold either - gold just doesn't turn black like that, not even in a fire (which is one property people have always admired about gold). Brass does turn black. But I'm not even entirely sure it's a coin. I don't see anything in that picture that's not explainable as random scratches.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis