I've heard that people use sand... but I just used stuff called "chick grit" - if you go to the feedstore, they should have grit in various sizes and they should note on the various sizes what birds should use what grit. One size will say it's good for chicks. If you sprinkle a little grit on their food starting at three days old, it helps stimulate gizzard development. It is not totally necessary to use it until they are eating more solid things than chick starter/crumbles and yogurt, but it's good to get them started with a little bit from an early age.
