The name given to iron pyrites, which looks a little like gold but is worthless.
Martin Frobisher returned to England from a voyage to find the North West Passage in 1576 with a cargo of this supposed 'gold mineral'.
The term has come to denote any apparent treasure trove that turns out to be worthless. The first reference I can find is an apparently ironic use in a headline in The Atlanta Constitution, June 1888:
The Search for Captain Kidd's Buried Wealth...
A party of men digging in a Connecticut cave - ploughing up New Jersey ground in the fruitless search.
That headline would suggest that the term was already known to the paper's audience, but I can find no citation of it prior to 1888.