Will, chicken grit is usually crushed granite. There is a form that is crushed oyster shells, I believe. But, that's not the one used by gardeners. It is more readily available in the south, as opposed to pumice. Both chicken grit and pumice are used in place of perlite, which is very light and has the tendency to gravitate to the top of the potting mix, leaving plant roots with little air space to breath. These two products help to alleviate that problem.
