I refer to the lumps of stuff outside a mine pit as "Slag". -- says Helen, the face that looked into a thousand pits.

I and, I think, most people this side of the pond would agree. Mostly, we Brits would imagine a coal mine, if you mentioned a "slag-heap", but I have heard the term used about the waste from other sorts of mine as well - it's just that we have (had, pre-Thatcher!) more coal mines than any other sort.