Chicken grit is crushed rock, whatever that rock may be. Depending on where you are located, it is most often made from the regional local granite or limestone. It can be oyster shells, although I couldn't really say if it is from oysters or some other shell species. It is usually sold in 50 lb. bags of uniformly sized aggregate: common sizes are #1,#2,#3,#4. Number 4 is often referred to as turkey grit, and number 1 is often referred to as chick grit, although "chick" grit is also the general shorthand for all the sizes.
