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.