A lustrous, grayish-white, strong, ductile metallic element obtained primarily from zircon and used in nuclear reactors because of its high resistance to corrosion, used in ceramic and refractory compounds, and alloyed with niobium, zinc, and other metals. Atomic number 40; atomic weight 91.22; melting point 1,855°C; boiling point 4,409°C; specific gravity 6.52 (at 20°C); valence 2, 3, 4. See Periodic Table.