Cerium is one of the most reactive lanthanide metals and it must be stored under mineral oil. The metal is silver/grey. The picture above shows a small cylinder of the pure metal, which was totally covered by a thin oxide layer. Some of the oxide layer was scratched away and immediately the metal was immersed in mineral oil, from which water and oxygen were removed by storing that oil over potassium metal.

The following picture shows the same cylinder of cerium metal, but the other, still oxidized side. The oxide layer has a remarkable blue color. A single scratch is shown, which is due to handling of the piece of metal with a small metal spatula. This also shows that cerium is quite soft.