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The grit gets ground down into sand-sized pieces and then passes on through their system, no matter how big or hard it is to start with. I have found a few larger pieces if grit in the gizzard when I process them, but usally very few. It's mostly sand.
The harder the rock used for grit is, the longer it will last. Granite is real hard and will last longer than a soft rock but it will not last forever. Commercial grit is usually granite, since it is plentiful, cheap, and does a real good job. If your native rock is something other than granite, they wil use that. I think the main reason I find mostly sand is that mine are using the native sandstone as grit. It gets crushed down into sand fairly easily. They can get by on sand alone if they need to. It doesn't have to be large pieces.
They can use limestone as grit if that is your native rock. Limestone is made up mostly of old sea shells, so it is almost the same thing as oyster shells, just morphed into rock. It is pretty hard but the acid will eventually break it down so it does not last a real long time. They'll just eat more of it. If they are using limestone, any oyster shell you offer will probably last a real long time. They get calcium from the limestone.
I have a large plastic yogurt container wired to the run fencing that I use for oyster shell when I have chickens below laying age and feed Grower instead of Layer. Layer contains a lot of calcium and they can also get calcium from some plants, from hard-shelled bugs they catch, and some other things, like limestone. Even feeding Grower instead of Layer, mine hardly ever touch the oyster shell. They get plenty of calcium other places since the egg shells stay hard. This yogurt cup would work for grit too, but mine have access to the ground so I don't usually give them grit. If I have to keep them restricted for a longer length of time, I will gather pea-sized and smaller gravel from my driveway and throw that on the ground so they can have the fun of scratching for it. Don't use gravel road or gravel driveway rocks for grit if your roads are salted for ice. They cannot handle the extra salt that well.
If I were buying grit, I would keep it in a container and not just throw it on the ground. They would probably find and use most of it, but even at $10 for a 50 pound bag, it would just go against the grain. But if you have a free source of grit, they will find most of it and use it, plus have the fun of scratching.
