20 Calcium (Ca)

atomic weight 40.08, melting point 839° C, density 1.54g/cc

Calcium medal by Metallium, Inc. Photo used by permission.
Diameter 19 mm, 0.6 grams, 99% pure. $42 on 7/16/2009.

If it were not sealed in a glass capsule it would corrode rapidly in air.