Terbium is a chemical element with the
symbol Tb and it has an atomic number of 65. The
atomic weight is 158.92535. 
Things you might not know about
Terbium:
- The largest consumer of the world's terbium
supply is in "green" phosphors (which are usually yellow).
- The brilliant fluorescence allows terbium to
be used as a probe in biochemistry.
- The richest current commercial sources of
terbium are the ion-adsorption clays of southern China.
See GC2XAY2 - Hydrogen, for an explanation
about this series of caches.