I will clarify why I was inquiring about using the oyster shells from the feed store. I am raising crickets. The cricket recipe that I use calls for powdered milk (to boost the calcium) but I chose to use Rep Cal calcium supplement because I had a surplus of it from a friend. My crickets eat A LOT! Which, in turn, makes me go through a lot of calcium supplement. Either way, milk or calcium supplement, can get expensive. I know that most people would not go through enough to make it worth their time and effort, but for those of us who are raising feeders and add it to their feeder chow I thought it might help with cost. Thanks to everyone who replies to this post.