What are the three types of grains of rocks?

The three types of grains in rocks are igneous, sedimentary, and metamorphic grains. Igneous grains form from the cooling and solidification of molten material, sedimentary grains originate from the accumulation of mineral and organic particles, and metamorphic grains result from the alteration of existing rocks under heat and pressure. Each type reflects different geological processes and environments of formation.