Rutile is the most abundant natural form of TiO₂. There are numerous polymorphs that include anatase and brookite. Iron (Fe⁺²) sometimes substitutes for titanium in some specimens of rutile. When this occurs, a valence difference between iron and titanium requires balancing – and that balance is often accomplished by substitution of niobium (Nb⁺⁵) and/or tantalum (Ta⁺⁵) for another titanium. Substitution of these elements increases the specific gravity of rutile and causes a black color in both the mineral and its streak.