These formations are found in only one location: Illinois coal mines, where they formed millions of years ago between layers of coal shale. Also known as pyrite dollars or miners’ dollars, these formations are found hundreds of feet beneath the Earth’s surface. Because the coal shale forms in sheets, pyrite (an iron-based mineral) caught between the layers has no room to expand into its usual angular shape. Instead, it fans out from the center into a round formation, which looks so much like a sand dollar that people originally thought they were fossils. Miners would carry specimens out in their lunchboxes as a curiosity to share with family and friends.