Moqui Marbles - Escalante Plateau. Hikers rambling through Utah's candy-striped canyons sometimes come across a strange-looking sight. Where the Navajo Sandstone loses its iconic peach, orange and red stripes, hundreds of round, iron-coated stones often litter the ground. The stony spheres are concretions — sandstone balls cemented by a hard shell of iron oxide minerals. Often called moqui marbles, acres of the chocolate-colored rocks are scattered across Utah and Arizona. They tumble from the pale, cream-colored Navajo Sandstone beds, when wind and water wash away the softer rock. For decades, the rocks were simply a geological oddity. Then, look-alikes were discovered on Mars (the so-called Martian blueberries). The milestone — among the early evidence for water on Mars — boosted interest in Earth's iron baubles. [Photo Gallery: See Fantastic Moqui Marbles]