pyrite

Fool's gold, in sharp cubes or as brassy nodules and crusts. Brass-yellow with a hard metallic glitter, and hard is the word: a knife will not scratch it, where real gold is soft enough to dent. Crush a corner and the streak runs greenish-black, never yellow.

Here it lives in the Antrim shale, Alpena best, most often as brassy nodules and crusts rather than clean cubes. A lot of those nodules are really marcasite, pyrite's twin with the same formula and a different crystal shape. Marcasite is the one that sweats and crumbles in a drawer, so keep it dry. Fresh it flashes; weathered it dulls to a rusty brown crust.