There are a variety of things that are often mistaken for minerals, but are not actually minerals. Some of these include: fossils, rocks, crystals, and gemstones.
Fossils are the remains or imprints of organisms that have been preserved in sedimentary rock. They are not minerals because they do not have a crystalline structure. Rocks are made up of one or more minerals, but they are not themselves minerals. Rather, rocks are classified based on their composition and texture. Crystals are solids with a regular, repeating internal structure. Gemstones are crystals that have been cut and polished for use in jewelry or other decorative items. They are not minerals because they have been altered from their natural state.