So natural phenomena can hurt them but constructed objects can't?
What if they fall on a pointy stalagmite? Do they take piercing damage? What if a giant is nearby and could potentially wield it?
If a tree falls in a forest onto a wererat and no one is around to wield it, is it a weapon?
The stalagmite could be wielded as an improvised weapon, so no damage. There aren't any rules currently about monsters are sufficiently powerful enough to overcome immunity to weapons afaik. So, based off what we currently have, no a giant would not do any damage with with a non magical non silvered stalagmite.
Anything else?