It is customary on Taanis Esther to give a coin (or three coins each) valued at half the local currency's standard unit to tzedakah in commemoration of the mitzvah of Machatzis Hashekel (Rema 694:1).
In America, there are a number of different half-dollars. Most common in circulation are silver half-dollars and copper-clad nickel half-dollars. Ideally, should one give the silver half-dollar?