Some bird grit has calcium added. They probably won’t eat enough to harm themselves but extra calcium is not good for growing chicks. I don’t know what other additives might be in bird grit. You can always check the label to see the ingredients. Different brands could be different.
If they have access to the ground, whether it is grass covered or plain dirt, they will find their own grit. I don’t know what your lanai looks like but it probably doesn’t have a dirt floor so, no, they won’t get grit from that.
Chickens will use whatever rocks they can find as grit. It doesn’t matter if it is granite, quartz, basalt, diamonds, obsidian, or something else. The harder it is, the longer it lasts before it gets ground down. You can normally find bags of “grit” in a feed store, some smaller pieces sold as chick grit and larger pieces sold for laying hens or pullets. This is made from granite, which is hard and lasts quite a while in their gizzard. They screen the wastes from granite quarries to get the different sizes.
For chicks you can use a coarse sand like construction sand or just get sand from a creek bed, something like that. If you get it from a saltwater beach, you should rinse it first. Chickens don’t handle extra salt real well, so you rinse the excess salt off of it. Adult chickens can use pieces of rock up to the size of a green pea as grit, but they can use smaller pieces too. Chicks of coarse use smaller sizes, but they know what they can handle.
