And I once was analyzing a guy's old arrowhead collection from Iowa - no info on where he picked up or dug stuff so scientifically useless, but there were a couple hard black lumps sorta like charcoal briquets. Couldn't figure them out, but touched to tongue, they tasted like powder, ground a bit off and touched a match to it, and it poofed right up. No idea where from or how old, but - interesting.