Many aviculturists give their birds shell grit or cuttlefish bone thinking it is a suitable grit. Cuttlefish bone certainly is great and beneficial for the birds, but it performs a totally different function to that of grit. Shell grit and cuttlefish bone are fine for the supply of calcium that a bird needs, but useless for the hard task of grinding up seed. The seed kernels that end up in the gizzard are hard and need more than small pieces of cuttlefish bone or even shell grit to break them down to tiny pieces.
Some aviculturists believe grit is grit and any type is as good as any other, but they fail to see what use ‘hard’ stones are. Hard stones are needed because birds have no teeth. After looking into a great many gizzards (the muscular organ that grinds the birds’ food into an acceptable product for them to use) I believe it is better to think of the gizzard as a “tumbler” or breaking down machine like the tumblers used at brickworks to knock sharp corners off certain varieties of bricks.
Another aspect to be considered is the size of the grit used, and this depends on the variety of birds kept. Finches tend to have stones of 1-2 millimeters in diameter in their crops; whereas King Parrots will have stones that are 2-5 millimeters in the crops they eat. These are the sizes of the stones selected by birds that have access to gravel or coarse sand. For your birds in captivity don’t make the mistake of using beach sand as it is too fine, fine even for finches. Use clean creek sand that has a high percentage of quartz. Birds on free range with access to all varieties of grit will almost always have the greatest percentage of grit consisting of quartz in their gizzards. On the scale of hardness in minerals, quartz rates a hardness of 7 as compared to diamond with a score of 10. Obviously, the birds favor the harder grit as it does a better and more efficient job of crushing up the seeds. The sharpness of the stone doesn’t seem to worry the birds either. Make sure to renew your grit as the birds must renew the supply to their gizzard. As the stones in their gizzards are worn down smooth and to a size that is of no further use in seed crushing, they are passed out through the bird in their droppings. A supply of fresh grit should always be available.