I wouldn't use sandstone, IMO...Limestone, in general, is not a problem....much of it is old coral anyway and it is commonly available in fish stores. It is generally white often with lots of holes. A good base rock.
But anything with color like "rainbow rock" or something I would stay away from in a saltwater tank. They are used in freshwater mostly. Limestone is often used in african cichlid tanks for the same reason it is used in marine...to keep pH high. But the color in some of the other sandstone "fish store rocks" comes from various minerals like iron. Sometimes good in lower pH planted freshwater tanks where they are usually used. It is a nutrient for plant growth. In addition they are silica based rock. Neither silica or iron or some of the other minerals are good in a reef or saltwater tank, IMO.
I'd run to the fish store and look for a hunk of limestone rock.