Thulium (Er) is a chemical element with atomic number 69. It belongs to the so called rare earth metals. The Earth's crust contains only about 0.5ppm (parts per million) of thulium, meaning 0.5 milligram per kg.

Thulium emits a blue light when exposed to UV radiation. Because of this, thulium is being used in the Euro bank notes to prevent counterfeiting.