All chickens basically require grit to help grind their food up and often insoluable grit is used (granite or crushed gravel) for this though crushed oyster shell serves this purpose too as well as being a source of calcium. Free range birds will usually find sufficient for this purpose from the ground.
Chickens also need calcium for egg production and if fed on a balanced compond feed young (pullets) will get this from their layers pellets but older hens may need additional supplements available ad libitum and crushed oyster shell is the best source. Again chx free ranging on chalky soil will find plenty themselves if they need extra from their feed.
Most people provide grit ad lib anyway as I do even for chx on grass, even though it's arguable that only chx not kept on free range ground really need it. Here's three types usually sold in feed merchants.
Insoluable - crushed gravel basically, sometimes just called grit
Soluable - crushed oyster shell (contains about 90% calcium) which also will provide a grinding agent too.
Mixed - often has oystershell, other crushed shell (crushed mussel would be ok as an mostly insoluable grit) and gravel. I use mixed usually and don't make it easily acessible I like to encourage my birds to be active and work for their living. That way they stay free from boredom and mentally healthy.
Don't as some do routinely mix it in their feed unless you are short term treating for some gross deficiency and then limestone flour is a more absorbant form of calcium to use for this purpose.
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HF