Grit is made primarily by grinding up minerals and sand. Some of the substances used are insoluble and cannot be digested. These include silicates and sandstone. Soluble, and therefore digestible, grit is primarily made of limestone, which is calcium carbonate. The soluble grit can be made from cuttlebones or from ground up oyster shells. Since the primary purpose of grit is to help remove husks and shells from seeds, birds that ingest whole seeds, like doves, would likely be the best candidates for needing grit in their diets.
To learn more, read Grit – What Is It and Who Needs It