Lutetium metal is silvery white and relatively stable in air; is the hardest metal of the rare-earths; used as important additive to some specialty alloy. The lutetium atom is the smallest among the lanthanide atoms, due to the lanthanide contraction, and as a result lutetium has the highest density, melting point, and hardness of the lanthanides. Some of these properties can be explained by its position in the d-block, which gives the metal properties of some heavier transition metals. Occasionally, lutetium is classified as a transition metal on this basis, although the IUPAC classifies it as a lanthanide. Stable lutetium can be used as catalysts in petroleum cracking in refineries and can also be used in alkylation, hydrogenation, and polymerization applications.