Constant dropping wears away a stone. -mid 13th; earlier in Greek

It’s an ill bird that fouls its own nest. -mid 13th

Constant dripping hollows out a stone.

The cobbler always wears the worst shoes

You can't get blood out of a stone

It's a little bit analogous to dropping a stone into a pond, and watching a ripple and then sometimes a second ripple go out from the center.