For calcium, chickens love egg shells more than oyster shells but they pass through quicker than the oyster shells. I can pour crushed egg shells in an oyster shell container and they will pick out the egg shells before touching the oyster shells. I don't see why.
For grit, granite doesn't...
Crushed egg shells are not really big enough or hard enough to be used as grit. They can be used as a supplemental source of calcium but they are not as good as oyster shells for calcium. A typical limestone pebble would work better as grit but granite is the best grit.
If your hen has sour...