orrect gem identification is crucial to the gem and jewelry trade. However, accurate information on a gem's origin rarely accompanies a stone from the mine, or follows a synthetic through the trade after it leaves its place of manufacture. Today, natural and synthetic rubies from a variety of sources are seen routinely (figure 1). Usually, careful visual observation and measurement of gemological properties are sufficient to make important distinctions (Schmetzer, 1986a; Hughes, 1997). In some cases, however, traditional gemological methods are not adequate; this is particularly true of rubies that are free of internal characteristics or that contain inclusions and growth features that are ambiguous as to their origin (Hänni, 1993; Smith and Bosshart, 1993; Smith, 1996). The consequences of a misidentification can be in the tens of thousands, and even hundreds of thousands, of dollars. Ruby is a gem variety of corundum (Al 2 O 3) that is colored red by trivalent chromium (Cr 3+). Besides Cr, most rubies contain other elements in trace amounts that were incorporated during their growth, whether in nature or in the laboratory. For the purpose of this article, we consider trace elements to be those elements other than aluminum, oxygen, and chromium. These trace elements (such as vanadium [V] and iron [Fe]) substitute for Al 3+ in the corundum crystal structure, or they may be present as various mineral inclusions (such as zirconium [Zr] in zircon) or as constituents in fractures. The particular assemblage of trace elements (i.e., which ones are present and their concentrations) provides a distinctive chemical signature for many gem materials. Since the trade places little emphasis on establishing the manufacturer of synthetic products, this article will focus on how trace-element chemistry, as determined by EDXRF, can be used for the basic identification of natural versus synthetic rubies. It will also explore how EDXRF can

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