Europium - Eu
Europium is one of the most reactive lanthanide metals. It must be stored under mineral oil. The sample, shown here, is oxidized, and covered by a green layer of oxide. The size of this sample is approximately 1x1.5 cm². The layer of oxide does not adhere to the metal and does not protect it. When the metal is not stored properly, then it is oxidized, the oxide simply falls off, exposing new fresh metal and in due time, the entire piece of metal is converted into a green little heap of oxide.
This sample is less oxidized on the other side. The picture below shows the other side, which also shows green oxidation products, but also a darker surface area.