Question about rocks.

My friend recently bought a bag of rocks for me from a craft store knowing I was gonna be attempting a new planted tank. But on the back it says not for use in aquariums, what about them would make it not safe for use? Would the be okay if I just washed them thoroughly?

Thanks in advance!

My friend recently bought a bag of rocks for me from a craft store knowing I was gonna be attempting a new planted tank. But on the back it says not for use in aquariums, what about them would make it not safe for use? Would the be okay if I just washed them thoroughly?

Thanks in advance!