There are two kinds of grit for chickens. Laying hens need both kinds. I found this, that explains it:
Grit refers to a finely ground hard substance that is given to chickens. There are two types of poultry grit: oystershell grit and flint grit. Oystershell grit is a type of calcium that helps make eggshells stronger. Insoluble grit, also known as flint grit, helps chickens with digestion.
Another name for the flint grit, at least here in the States, is crushed granite grit. This goes into the gizzard where it crushes the food, especially whole grains, as chickens lack teeth for grinding. Oyster shell does not perform this function. These should be kept in separate containers so the chickens can help themselves.