Planted Tank Guru
Join Date: Jul 2010
Location: Huntington Beach, CA, USA
I would assume those are a pretty soft type of limestone. Looking closely, can you see any of the little fossil critters which they are made out of? If they were collected along a shoreline, the rough texture might be something that is left of algae and might scrub off after a bit.
If they are local rocks, and your water is already hard and alkaline, there is no reason to think they wiill change your water much. If they are local rocks, people tend to forget that is what the water spends much of it's time in already. If you take them to some place where the water is PH 6.5, it will change the water more but fish and plants can handle that if you are not picking the fussy types.