Columbia Encyclopedia World Factbook American Heritage Coll. Thulium is a soft, malleable, ductile, lustrous silver-white metal. It is one of the rare-earth metals of the lanthanide series in group IIIb of the periodic table . It does not tarnish rapidly in dry air but should be protected from moisture. It forms compounds with oxygen and the halogens, most of which are light green.
Thulium is the least abundant of the rare-earth metals.
It is found in the minerals gadolinite and euxenite and in monazite, the chief commercial source. The metal can be obtained by chemical reduction of its compounds. The pure metal and compounds have few commercial uses, but they are often used without purification in combination with the other rare-earth metals and compounds, e.g., in lighter flints and carbon electrodes for arc lighting. Thulium was discovered in 1879 by P. T. Cleve. The Columbia Encyclopedia, Sixth Edition.