It is usually made from calcium-high materials; the calcium helps the bird in egg shell production.
[1] Other uses for shell grit include protecting plants from slugs or snails[2] and for aquariums.
Juvenile birds will often ingest smaller pieces of grit than adults, as in Sarus Cranes.
Grit that starts off in rough or angular pieces may become rounded off as it is used in a bird's gizzard.
For waterfowl and gallinaceous birds, there have been many avenues of research: their status as game birds, their population densities, any potential role of grit in lead poisoning, and pesticides that grit may include.