I'm in the USA, where it's easy to buy "grit" (little rocks) and oyster shell (for calcium) separately. So I have never personally had to deal with a choice like that.
In your case, there's a good chance the rooster can pick out suitable bits from the mixture, but providing the gravel/sand from a stream is also fine and might be helpful.
But you might not need to provide grit at all:
--If your chickens have access to gravel or small rocks outdoors, they can get their own grit.
--If your chickens are only eating commercial chicken food (ground up and then formed into pellets), they don't really need grit because the grinding has already been done for them.
(You providing grit will not hurt them in either case. They just would not be need it.)