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.
This thought got me thinking about sources of quartz to use for grit. My flock have been crossing through the woods into a little holler to pick for grit and I'd prefer they not go that far out of the meadow, so I decided to find them some quartz for grit. Found some 4 lb bags of quartz at Lowe's in the garden center. They went through a 4 lb bag of quartz in under a week and repeated that not long after when I got them another one. That's a lot of rock for a flock of 14 birds and I'm not seeing any in the stool, so not sure what to think about that.
I'm not seeing much difference in their intake or weight with the addition of more grit to their diet, so not sure I'll keep repeating that process too often, but have an extra bag of quartz on hand for the next time I see them using the holler a lot for picking up quartz.