Not sure what redstone is. Seashells (or any shell material) is not suitable as nonsoluble (digestive) grit for chicks, as it is not hard enough to last and contains too much calcium.
Hi rosemarythyme, this grit contains 23% Ca, so definitely not too much, and certainly better than feeding them crushed eggshells (I actually consume those and find them to be not only highly beneficial but also delicious), which contain roughly 40% Ca.
I looked up the composition of this grit, and here's what I could find:
"
Composition: Redstone, seashells, oyster shells, limestone, stomach gravel, flint, charcoal, anise (0.2%)
Analytical constituents: Crude ash 98.5% (41% ash insoluble in HCl), calcium 23%, sodium 0.15%, phosphorus 0.02%".
And here's the composition of the grit specifically for chickens that the same producer sells (not usually available in my area):
"
Composition: marine shells, flint, oyster shells, charcoal
Analytical constituents: crude ash 97.4%, lysine 0%, methionine 0%, calcium 19%, phosphorus 0.022%, sodium 0.21%.”
Manufacturer does recommend the former for pigeons/doves only, however this is the grit of choice for chicken growers in my area. My own adult birds seem to like it a lot, too, but they're free range and also have access to plenty of soil and other grit they find in my orchard.
From what I could gather, the main concern with this pigeon grit being used for chickens is redstone not being hard enough as granite grit or flint, which the chicken-specific grit contains a little more of. Aside from that, they're basically the same thing.
So I guess my thought on the matter is: if you have access to chicken-specific grit, which contains slightly more flint and 4% less Ca, by all means go for it. If all you can get your hands on is pigeon grit, with more redstone and a little less flint, that also works just fine.