I am not saying that filtration rocks, instead I am talking about rocks being used as water filters. Crazy? Well, not really. Rocks are an important part of water filtration technology and can serve different purposes. Bowling ball size rocks will not do much filtering, but as we get smaller and smaller particle sizes, down into the sand range, rocks can actually help filter sediment and dirt out of water.
Garnet is a rock that can filter down to about 35 – 20 microns, which is a lot less expensive than going through a lot of cartridge filters. Gravel is commonly used as an under-support for the outlet screens of big water softeners, and when small enough can filter out large sediment particles. Limestone will actually help remove acids and raise water pH. Because they it is porous, sandstone can help trap smaller dirt particles. Dolomite also has use as a coarse filtration media.
Finally, the greatest water-filtration rock of all time, lava-rock is great for removing iron. By running water over lava rock, in a waterfall style filter, all the crevices, nooks and crannies in the rock causes the water to come into contact with more oxygen, and accordingly the iron precipitates out as iron-oxide (rust) which can be then filtered out with a second stage filter.