Grit can be any stoney material. Ground/crushed granite, sand, Tiny pebbles...etc. They swallow the grit and it deposits itself into the gizzard. The gizzard is a muscle sack. The gizzard contracts repeatedly; grinding any seeds, corn, or hard food. Thus making the food into a meal or pulp. then the pulp/meal is sent on down the digestive tract and absorbed easier by the bird. You can use clean coarse sand or pet bird grit for chicks. If you cannot get chick grit. I use the granite. I just sprinkle a lil bit on the feed atleast once a week. When they go outside, the birds will pick up most of the grit they need from the ground. Once they get a lil older you can give them the grit free choice in a container. As chicks they sometimes confuse the grit as a treat and can die from it compacting in the digestive tract. Just make sure they don't get stuffed with grit.
Good luck and God Bless!