Hello; No idea about the rocks being suitable as far as their chemistry, but do wish to pass on a caution. I am going to assume the tank has a glass bottom. I have read of the bottom glass being shattered by rocks.
One fellow had some rock stacked and one fell from it's position. Shattered the bottom of the tank and flooded the fish room.
If the tank is acrylic I am not sure how they hold up.
If a glass tank I would consider placing a sheet of plastic/acrylic on the bottom, then the gravel/sand and then the rocks.
I would also consider bonding the rocks together with silicone. Of course the rock will have to be clean and dry. I have used silicone to hold slate in place before.
Good luck