There are actually two types of chicken grit available on the market. Flint grit or insoluble grit is used for grinding down food and Oyster shell or eggshell grit (they have the same effect) – this is a source of calcium to help the chicken make their eggs stronger
What is Flint or insoluble grit?
As chickens find it hard to eat non-grinded food, we grind it down for them – they use a strong muscular organ called a gizzard. The chickens, whilst foraging, will pick up the grit – it is kept in the gizzard which is later ground in the grinding process. If the chickens are kept free range then they should be able to find enough of this on their own, but nowadays not everyone is able to afford things as space, or even security from predators which would allow them to do this naturally.
Flint, or insoluble grit is much cheaper and available from most pet or farm shops. The container that you put the grit in may cost a bit more, but it would last.
Oyster shell or soluble grit?
In order for the chicken to make strong eggshells, they are required to maintain a certain amount of calcium in their diets – most of an eggshell is made from calcium. Nowadays, The research has been done for formulated feeds, it isn’t so critical to provide oyster shells due to the fact that layer feeds already have a sufficient amount of calcium in them. However, you can mix this with flint grit – this is a cheaper alternative.
High production hybrid hens are capable of producing an overwhelming number of eggs each year on a very little feed, so they are more likely to need a bit more calcium to produce the eggs. You can buy oyster shells at your local store, otherwise if can’t, you can just bake crushed eggshells – they’ll do the same job as they are made out of calcium themselves.