the problem with the "river stones" is this, granite one of the primary rocks for finding quartz in, leaches Arsenic into water, Pure quartz is inert (99% non-soluble in water) but the minerals that surround the quartz can be very deadly to marine life. any reds in stone usually indicate non-chelated Iron (very dangerous in aquaria). greens can indcate tons of minerals including the ever deadly copper. There is any mix of minerals that while fairly harmless in a free flowing riverbed may devastate you marine tank or just cause an unsightly algal bloom, or not even that much, just occasionaly kill something here or there, or in the case of arsenic with good water changes all you might ever notice is that your biological cant keep up with your ammonia because arsenic will kill your biological bacteria, and on, and on..... chances are you may never expirience any problems if you put those in your tank, BUT you could also never stop haveing trouble. its not worth the risk,
if you want something smooth to put corals on for propagating use WHITE marble, (mostly calcium carbonate) I must stress the importance of using only white marble colored marble is colored from mineral deposites in the stone, this is/can be very bad.