Gadolinium, chemical element, symbol Gd; for physical constants see Periodic Table. Gadolinium, a rare-earth metal, was discovered by J.C.G. de Marignac in 1880. It occurs in bastnasite and monazite, which are the principal sources of the element. The metal is prepared by reducing the anhydrous fluoride with calcium. Gadolinium is silvery-white, soft, malleable, and ductile. It has the highest neutron capture cross-section of any element. Gadolinium is strongly magnetic but loses this property on heating. Gadolinium and its compounds are used in microwave applications and as color TV phosphors.