concretions

Iron-oxide concretions – curious-looking dark spheres – can be found by the hundreds of thousands in the dramatic red and white outcrops of Navajo Sandstone in Utah. The concretions typically consist of a sandstone core surrounded by a thick iron-rich rind, and they have an interesting geologic history and Native American cultural history. They also resemble Martian spherules or "blueberries", and may share similar origins.