I posted on the chick board about this, actually. I happen to have a gravel driveway made from granite scrap, several colanders, and some sand. I made a little plastic tray up with a section of sand, a section of small bits of driveway gravel that could pass through a metal screen colander, and a section with bigger pieces that could pass through a plastic colander with fairly large holes. I then put in the tray and pulled up a chair to watch. The smallest chicks chose the small driveway gravel and the bigger chicks chose the large driveway gravel. None of them ate the sand. So, I think they'd prefer something coarser.
I didn't want to free range peeps but I also didn't want to buy grit when I have a ton of it right outside. I expect that once they're big enough to range, they'll collect their own driveway gravel pieces.