I use eggshells a lot, I boil them for 15-20 minutes to make sure everything is cooked off then put them in a coffee grinder on fine. You may still need to put then in a freezer type zip lock bag and smash the crap out of them with a hammer (which I do and its great fgor taking care of some anger issues lol)
I've heard of mixtures like this, crushed eggshells and a pinch of fine sand and what not. Its supposed to be a good source of calcium and I guess a form of grit which helps break down thick insect shells in the stomach. Kind of like how crocodiles eat rocks to help grind up their big mouthfuls of food, just on a smaller scale. I think VMS herp's site had a recipe for this "grit" though I could be wrong.