Base metal is a catch-all term in the jewelry industry for metals used in costume jewelry. In metal working, base metal is any metal that is not one the noble or precious metals. Precious metal usually means Platinum, Gold or Silver. In costume jewelry, base metals are often plated with gold, silver, nickel or rhodium. Many plated items are plated first with copper.
Many gold plated items have a white nickel plate under the final gold plate.
Pewter includes any of the numerous silver-gray alloys of tin with various amounts of antimony, copper and sometimes lead. Because of health hazards, lead is seldom used in pewter today. Pot metal and white metal are terms for tin based alloys used in low temperature casting of costume jewelry components. White metal castings are usually three-dimensional rather than flat and are often plated. The exact composition of white metal varies, because each casting foundry and shop uses their own proprietary formula.