Red Rocks in Botton of Run?

Recently I visited a farmers-owned feed store and they had a beautiful chicken coop out back where they used a red colored rock from Purina...almost like lava rock in the bottom of their coop. They said they've been using it for 7 months now and it's been great and the clean up is super easy because all you have to do is spray the rocks off with a hose/water and the poo goes straight into the ground. We are beginning to work on our coop for our 8 girls this week and I'm considering using this idea but I can't find any other reviews on this topic? Anybody else out there try this and if so,what are your thoughts on odor, cleanup, and overall well-being of the girls standing on this for hours while I'm not home and they can't free range? Thank you!