crush oystershell may do well as a dual purpose...good for eggs and calcium intake and im sure it acts as grit in the crop before leaviing the crop. I've always wondered about that...but anyhow if it does act as both, then id' say crushed oystershells...because theres a bunch of peices in there small enough for my quail and also big enough for my chickens.
There are some special grits just for pigeons that have calcium in them. I don't think oyster shell counts as a 'grit'. I use a starter grit from my feed store, and then supplement with oyster shell. If you used layer feeds, you wouldn't need oyster shell at all. The starter grit is small enough for baby chicks and button quail, but still large enough for bigger birds.