Colorless or faintly yellow metal, that tarnished in moist air .
In nature, gadolinium is found in the Earth's crust, 6.1-6.36 ppm, in minerals such as samarskite, gadolinite (ytterbite), xenotime and other rare earth minerals . The isotopes found in nature are 152Gd (0.20 %, T½ = 1.08 · 1014 years), 154Gd (2.18 %, stable), 155Gd (14.80 %, stable), 156Gd (20.47 %, stable), 157Gd (15.65 %, stable), 158Gd (24.84 %, stable) and 160Gd (21.86 %, stable) .
In pure form, gadolinium exist in two forms. An α-form (hexagonal close-packed crystal) which changes to a β-form (body-centered cubic crystal) at >1262 °C .