So far on one lawless island that I managed to get setup on, I've found the above rocks, along with the normal larger standard rocks (sandstone mostly in this area).
I first saw the Nitre, as it was most visible, then noticed the other types of rocks, and stumbled upon the Metal node. It's possible that they all spawn near each other, so if you see any (the Limestone seems most prevalent), it's worth looking for more.
They all harvest between 50-70 Metal with a metal pick, less so with stone (can't remember and don't want to waste it by testing).
Seem to respawn in the exact same space, and look exactly the same each time.