The Flint Grit (Insoluble) is essential as it aids the grinding up of food in the gizzard in effect acting as hens "teeth".
The Oystershell Grit (Soluble) which is a source of Calcium is absorbed in to the body and is important in the formation of egg shells as well contributing to the well-being of the bird.
Mixed Grit is an essential part of a Chickens diet.
Comes in resealable tub which helps avoid contamination.