There are two reasons the grit we buy is granite. Granite is extremely hard, some larger pieces may last for a month in the gizzard. The other reason is that it is inexpensive. Practically all the grit we buy is waste from granite quarries. They sift the debris through screens to get the right sizes and bag it. So it becomes part of the revenue stream instead of just more waste they have to deal with. It’s still a very minor part of the overall wastes.
The rock the chickens eat generally stays in the gizzard until it is ground down to sand. Then it passes through their system and out the rear end. How long it lasts depends on how hard it is and the size of the rock to start with. The rock grinds against other rock as well as the walls of the gizzard and what it is grinding. Some rocks, like limestone, are dissolved by the digestive juices. I grew up in East Tennessee where a lot of our rock was limestone, we never had to offer any calcium supplement. They got all the calcium they needed for hard shells by the rocks they ate. They ate other types of rocks for grinding. Some rocks, like sandstone, crush pretty easily and reduce to sand-sized particles pretty quickly. How much grit they need to eat depends on what rocks they find.
There is a common theory on here that grit needs to be sharp and angular. It makes sense that sharp angular rocks will grind up things faster. But anyone that has butchered chickens, opened the gizzard, and paid attention to what is in there will quickly see that grit is really smooth. Like rocks in running water or in wave action, the sharp points are quickly worn away. They may not be perfectly round but they are smooth. For anyone that has seen a mill where grain is ground into flour or meal, how sharp and angular are the millstones? Or it you have a mortar and pestle in your kitchen to grind up spices, how sharp and angular are those? Even without sharp angular points they still grind.
If our chickens have access to the ground most of us do not need to supply grit, or as the Brits among us call it, insoluble grit. But if you are on certain soil types like the drained dried swamp muck in South Louisiana or they have access to a small portion of the ground where the available rocks might be used up, offering insoluble grit could be a good thing.
