Out of all the kinds of rock available to my free range flocks down through the 40 yrs I've kept and butchered chickens, there is consistently one kind of stone I find in gizzards across the board...white quartz. That seems to be the preferred stone of all kinds available, even when actual commercial grit is supplied. That is also what we find in wild turkey gizzards. Not sure if any studies done on that but that's the one stone that I always find more prevalent than other kinds of rock. Second to that are wild cherry pits. Long, long after cherries have been eaten and are no longer to be found, I'll butcher and find cherry pits. Not because they cannot be digested and passed along...if that was the case the birds would all die from impacted gizzards.