Grit is good, But i also bake egg shells for mine. I place them in the bottom of the oven on baking parchment, and bake when the roast is in, for about half an hour or so. They come out very brittle and so easy to crush. The hens have no idea they are eating shells, and it doesn't give them the idea to peck their own eggs.
You will occasionally get the very odd plastic egg, and on the very rare occasion you may get an egg the size of a malteser, this is not uncommon either.