3e SRD:Holy Symbol
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Holy Symbol, Silver or Wooden
A holy symbol focuses positive energy. Clerics use them as the focuses for their spells and as tools for turning undead. Each religion has its own holy symbol, and a sun symbol is the default holy symbol for clerics not associated with any particular religion.
A silver holy symbol works no better than a wooden one, but it serves as a mark of status for the wielder.
An unholy symbol is like a holy symbol except that it focuses negative energy and is used by evil clerics (or by neutral clerics who want to cast evil spells or command undead). A skull is the default unholy symbol for clerics not associated with any particular religion.