Here is a brief description of terbium.
Terbium is reasonably stable in air.
It is a silvery-grey metal, and is malleable, ductile, and soft enough to be cut with a knife. It is a rare earth metal found in cerite, gadolinite and monazite. The element itself was isolated only recently. Here is a brief summary of the isolation of terbium. Terbium metal is available commercially so it is not normally necessary to make it in the laboratory, which is just as well as it is difficult to separate it from as the pure metal.