Shells from the coast of Maine. They are a form of calcite called aragonite. These shells used to have little animals living in them. When the animals died, their shells washed up onto the shore. If the shells were covered with sand and dirt and maybe other shells and buried deep in the earth for a long, long time, then they might become fossils.
Fossils are traces of life from long ago.
Fossils can be formed from shells, or plants, or even insects. Usually, they form from something that was hard, like shells or wood. The original material goes away and may only leave a print in the rock, or it might be replaced by some other type of rock. These shells have been replaced by limestone, but you can still see the shape of the original shells. These fossil brachiopods came from the top of the Jemez Mountains in central northern New Mexico.