Iron-grey, ductile mallable metal .
In nature, cerium is found in the Earth's crust, 46-66 ppm, in minerals such as monazite, bastnaesite and cerite, of which the first two are the commercially important sources . The isotopes found in nature are 136Ce (0.19 %, stable), 138Ce (0.25 %, stable), 140Ce (88.48 %, stable) and 142Ce (11.08 %, T½ = 5 · 1016 years) .
In pure solid form, cerium exist in the following forms:
α-form: face-centered cubic crystal at < -150 °C 
β-form: hexagonal crystal at -150 to -10 °C 
γ-form: face-centered cubic crystal at -10 to 730 °C 
δ-form: body-centered cubic crystal at > 730 °C