Graphite is the mineral most commonly used to make pencils. It is a form of carbon and has properties that allow it to easily leave marks on paper. The structure of graphite consists of layers of carbon atoms arranged in a hexagonal lattice, which allows the sheets to slide over one another easily, producing the characteristic black mark that we associate with pencil lead. Pencils are often produced by encasing graphite in wood or other materials, allowing for a convenient and portable writing instrument. While there are also mechanical pencils that use plastic or metal casings, the core remains largely made of graphite.