If you look closely, you will see that the ground is covered with pieces of obsidian, or volcanic glass. This is out in what is called the Western Utah Black Rock Desert. We like to drive the back roads and find rocks and minerals. Technically, obsidian isn't a rock since it is made of silicon and is a hyper-cooled liquid but we pick them up anyway. We haven't ever figured out what to do with the rocks. I suppose our children can throw them away when we die.