Abstract: Both treated and synthetic gem materials are encountered today in the jewelry marketplace in increasing varieties and quantities. In some cases, they exhibit appearances that correspond closely to those of valuable natural gemstones. Although often displaying distinctive gemmological characteristics, some treated and synthetic gem materials can be difficult for jewelers to identify, especially when they lack gemmological training or access to standard gem-testing methods and equipment. In such instances, testing at a professional gemmological laboratory is necessary where the use of advanced scientific instrumentation by trained staff may be required. Accurate gem identification is essential to maintain both the commercial value of natural gemstones as well as consumer confidence. The goal of most current gemmological research is to provide practical means of gem identification for jewelers and gemmologists to help insure the integrity of the international gemstone market. Examples are cited of several treated and synthetic gem materials that currently present identification challenges.