What is devil toenails?

Devil toenails, also known as pyrite nodules, are small spherical structures composed of pyrite (fool's gold) that form in sedimentary rocks. They can be found in many parts of the world, including the United States, Mexico, Spain, and Russia. Devil toenails range in size from a few millimeters to a few centimeters in diameter and have a distinctive shape, resembling a toenail or a claw. They often occur in clusters and are formed through a process called concretion, in which minerals precipitate and accumulate around a central nucleus. Devil toenails are often used by fossil collectors and rockhounds as they may contain fossilized remains of ancient plants or animals that were trapped inside during the formation process.