Sevenless wrote:Sounds like bog iron to me.
Question remains: where'd that come from?
It wouldn't be the first surface metal source we've found.
Edit: Yeah looking it up I can't find any references to "heavy dirt", at least in english. I'm going with bog iron

You can get pretty much any ore both above and below ground.
Can confirm the following appear as boulders above ground. Cassiterite, Malachite, Heavy Earth and Black Coal (black ore does too, if it is indeed an ore).
Have found most of those in caves too and others that I've not found boulders of yet.
One thing to note though is not every ore is good. Iron Ochre for example gives iron, but it's about a 1% chance or something retarded.