Grit can be anything they can safely swallow, that isn't toxic, and that is harder than the food they are eating.
Mostly I use oyster shell for my laying birds. Yeah it's not as hard as granite, but it is harder than moistened grains which is all that it has to be. The gizzard doesn't care. It grinds them all together.
For reasons I've never understood it's hard enough to find simple oyster shell around here. Granite grit is next to impossible unless you want to buy it in those little pet sized bags. There is only one feed store that carries large bags and that's only because some folks at the university want it. I use it with my non-laying birds since they aren't going to be eating oyster shell. "Pigeon grit" is what they call it and that's the only size I've ever been able to find in a several county area.
Alternatively I use aquarium gravel which I can find easily enough. Be sure to get the non-coated, non-dyed natural gravel in a size that's appropriate for the bird usually somewhere between very coarse sand and extra fine gravel.
.....Alan.