Costume jewelry is jewelry that is made of less valuable materials, including base metals, glass, plastic, and synthetic stones, in place of more valuable materials such as precious metals and gems.

While at one time the manufacture of fashion jewelry included the use of inexpensive glass components - most notably rhinestones, or a manmade material known as "lucite" - all set in a base metal of pewter, nickel or brass, today the category also includes high end crystals, simulated gemstones, CZs or cubic zirconia simulated diamonds, and lab created gemstones as well as some semi-precious stones and sometimes wood. Metals include gold-over-brass, silver-over-brass, and sometimes gold-over-silver (known as vermeil), as well as sterling silver. In lower priced fashion jewelry you may still find gold plating over pewter, nickel or assorted other metals, and sometimes fashion jewelry imported from countries outside the United States may contain lead.