Tank decorations

I have some stone (not sure what kind, it's pinkish and looks stratified)
decorations left over from another fish tank. They were almost completely
covered in algae. The only way I could think to get it off (scrubbing didn't
work) was to put them in a garden fountain I have (I put chlorine tablets in
its water to prevent algae). It seems to have completely killed it, but how do
I make them safe to go in the tank? Would boiling work?