Thulium 's a chemical element that's the symbol Tm and atomic number 69. A lanthanide element, thulium's the least abundant of the rare earths. It's an easily workable metal with a bright silvery-gray luster and can be cut by a knife. It's some corrosion resistance in dry air and good ductility. Naturally occurring thulium's made entirely of the stable isotope Tm-169. Applications Rare and expensive, thulium doesn't have many practical applications. Thulium has been used to create laser lights,… (More on Thulium)