Allotropy (so named by J. J. Berzelius in 1841) refers to the existence of a chemical element in two or more distinct forms having different crystalline structures and/or physical properties. Allotropes may differ with respect to density, melting point, molar volume, color, and other physical properties. In some cases, there is a reversible, in others an irreversible transition from one allotrope to another.
carbon: chaoite , graphite , and lonsdaleite (hexagonal); diamond (isometric).
sulfur : native or α-sulfur (orthorhombic); γ-sulfur or rosickyite (monoclinic).
phosphorus: white/yellow (two forms: cubic and orthorhombic), violet and black (thus, four allotropes of contrasting properties); red phosphorus is a mixture.
tin : white (tetrahedral); gray (cubic).
iron: α-iron or kamacite (body-centered cubic and magnetic); γ-iron or taenite (face-centered cubic and nonmagnetic); δ-iron or ferrite (body-centered cubic and stable only above 1400°C).