Grit for Chickens: Information about the two types of grit ...
Flint grit or insoluble grit which is used for grinding down food and Oystershell grit, a source of calcium to help form strong egg shells. Flint or insoluble grit Chickens don't have teeth, apparently, they are very rare, so to grind down their food, they use a strong muscular organ called a gizzard.