When I used to raise pigeons for a hobby years ago, the grit I used to feed
them was a crushed seashell material. I think it was mostly pulverized oyster
shell, but sometimes I would find smaller and even micro shells in the grit.
It was the only grit sold in my local farm-town for chickens, etc. My birds
preferred this grit over any other material for their gizzards, and sometimes
would gorge themselves with it.

Scott
Florida