I find sandstone to be a very safe choice if it is hard enough not to drop pieces in the tank too much so that it becomes a nuisance. You might also be aware that fish have a habit of doing things their own way! I put things in for them and they lay their eggs on the heater or filter intake. We just are not that good at knowing what fish want! They get bored and decide to mess with my mind, maybe?
I use mostly sandstone and limestone, as well as a few other rocks and stones I've found in the deserts and mountains near my home, including quartz (spearpoints/arrowheads), raw gemstones (sapphires/garnets/topaz/onyx/amethyst), fossils and petrified wood....and they haven't affected my water quality or my fish at all.
I did read extensively on the subject before tossing my various rocks into my tanks, and found that most rocks are just fine. Including rocks that contain OXIDIZED iron (better known as 'rust'...this causes the red lines through some sandstones). The oxidation process makes it harmless, unlike unoxidized iron which is very toxic to fish. So, you should be just fine with sandstone, but watch out for "fool's gold" (pyrite). It should not be found in sandstone deposits but if you see other 'neat' rocks while you're gathering your sandstone, just beware of it. Pyrite contains iron which CAN leach into your tank and kill your fish. So any rocks with sparkly silver or gold flecks should be avoided.
I bring my rocks home, rinse them in tap water and scrub them with a hard brush to get any debris off of them, soak them in bleach water overnight and let them dry out for several days before putting them into my tanks. Thus far no problems at all.