IIRC Gizzard stones are a byproduct of butchering birds, which deliberately swallow grit or stones to grind stuff up in their gizzard/gastric mill which is passed food from the true stomach that takes it from the crop where it's temporarily stored, they need the stones and grit as they can't really chew and swallow stuff whole with the largest birds like ostriches and other ratites sometimes swallowing really big stones (also a significant number of trees have fruit with enormous seeds that rely on foraging cassowary swallowing them whole and passing them undamaged). I believe they count as a type of gem and they can come in every stone type which I think also includes metal ores/native metals. I used to keep chickens and they'd seek grit out, with crushed up seashells serving doubly as gizzard stones and a calcium source.

Dwarf fortress, Bukowski, Tax Cudgel lol!? Ok, I subbed! I guess youtube knows me better than I thought.