Chicken grit is from granite rocks, ground up into small particles about the size of very coarse sand. Granite is most often used for grit because, unlike limestone, granite will not dissolve in water. It is swallowed by all birds in the bird kingdom. It travels from the crop to the gizzard where it acts like a garbage disposal to grind up food so the bird can digest it.
Without grit, a chicken would eventually die of starvation. Grit eventually becomes rounded and worn, just as garbage disposal blades do, and it needs to be replaced. The chicken poops out the old grit and consumes fresh grit. Egg shells and oyster shell will dissolve in water so they are useless as grit.
The good news is that you can buy it at the feed store where you find the chicken feed.