I agree that granite is not necessary. While it may be superior in quality, most areas have hard stones available that meet the gizzard needs of a flock of birds, as long as they have access to the bare ground. I question if even when confined to a deep composting litter run that they would find adequate grit. If they can free range, they most assuredly will in most areas. I scoop washed gravel from my driveway. It is composed of varying sizes of stone, and the chicks go to town on it when ever I put it in their brooder. I see to it that they get a fresh scoop every week. I can easily increase the particle size simply be selectively choosing from my driveway.