Terbium is fourteenth in abundance among the 17 metals usually counted as "rare-earths", present in the earth's crust to the extent of only 0.9 ppm (about 1 teaspoon in every 63 tons of earth). Named for the Swedish village of Ytterby, the metal was discovered in 1843 by Mosander (along with erbium).
Small amounts of terbium are used in special lasers and some solid state devices. The monazite sand from which terbium is generally extracted contains only about 0.03% by weight of Tb.
Stephen R. Marsden (ChemTopics)