Thulium is one of the elements, near the end of the row of lanthanide metals. It is a metal with a nice dendritic crystalline structure. The picture above shows how the pieces of metal consist of parallel dendrites, and also a single dendrite is shown at the right. The metal is quite reactive and quickly dissolves in dilute mineral acids, but in dry air it is quite stable and remains lustrous for a long time. This sample has a purity of 99.9%. The size of these pieces is approximately 1 cm along their longest dimension.