Pewter won't hold mana. Mana is a Polynesian word for the energy that flows through all living things. It is this energy that mages harness and use to empower talismans and other magickal objects. Certain materials conduct magickal energy better than others. Interestingly, these are usually the same materials that conduct electrical energy well.
Copper is one of the best conductors.
Silver and gold are also good. Pewter, however, will only hold a charge for a brief time. Mana just drips off it. In the end, the best metals to use for jewelry are silver and gold.