Cobalt

[Rennhack]

A gray, hard, magnetic, ductile, and somewhat malleable metal, cobalt is relatively rare and generally obtained as a byproduct of other metals, such as copper.  Its most common radioactive isotope is cobalt-60, which emits beta particles during radioactive decay.

[Rennhack]

http://www.epa.gov/radiation/radionuclides/cobalt.htm