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Other than the comment on the oyster shell, you are getting good advice. If they free range, they are getting enough and they are eating things that they need grit for. In the brooder before they free range, I give them sand and small pieces of gravel from my gravel road and driveway. If yours is salted for ice in the winter, don't do this since they cannot handle the extra salt. Coarse sand, like construction sand, works well too. Don't use the play sand. It is too fine and smooth to do much good. Play sand will work and will not hurt them, but it will go right through them.
As Moxiechick said, the calcium in oyster shell is bad for them. It can damage their liver. Also, oyster shell is not really hard enough to use as grit. Many of their treats are harder than the oystrer shell and will grind up the oyster shell instead of the other way around.
Good luck!