In regard to Tom Wheaton's question about worn shard sizes - I  imagine the
size of the stone depends on the size of the bird.  I'm sure there is an
extensive literature on gizzard mechanics.   I did hear from someone just
today who once saw a sparkplug come out of an ostrich gizzard...
    I think a centimeter is an outside size for the worn ceramic frags I've
seen - off the top of my head I'd say most are more like a half centimeter.
                     Best
                     Larry McKee