Slate from outside safe to use to sink driftwood?

i got a piece of slate that sat around the edge of my friends fish pond and it had some green algae on it. I let is sit in tap water and scrubed alot of it off, but I don't know if i should put it in my tank to sink the driftwood. My friend used a piece of slate from his pond to sink his driftwood and he hasn't had any problems, but i don't know what he did to clean it.