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Black pearls have been found in Mexico's Gulf of California since the area was settled more than 2,000 years ago. Attempts at culturing pearls in this area have met with varying success over the past century. Today, a pearl farm in Guaymas is producing commercial quantities of mabe as well as bead-nucleated full-round cultured pearls from the indigenous pearl oyster Pteria sterna. This article provides an overview of the history of natural and cultured pearls from Mexico, describes pearl culturing at the Guaymas farm, and focuses on the properties of bead-nucleated cultured pearls from P. sterna. These cultured pearls have a brown or gray to dark gray bodycolor with various interference colors caused by the stacking of platy aragonite crystals and organic matter. One indication of their natural color (and their Mexican provenance) is a red fluorescence to long-wave UV radiation.

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colored strands marketed today may include a mix of cultured pearls from the P. maxima and P. margaritifera (Federman, 1998b) or consist only of cultured pearls that originate from the P. margaritifera. The latter may include several in the following yellow hue range: yellow, greenish yellow, brownish yellow, or grayish yellow. This makes some of them difficult to distinguish from similar-color cultured pearls from the P. maxima.Identification of the mollusk species in which a pearl was cultured is becoming an important issue in the industry. Until recently, it was relatively easy to identify freshwater, South Sea, "Tahitian," or Akoya cultured pearls just by size, shape, and color. Today, however, there is considerable overlap in these once distinctive characteristics from one type of cultured pearl to another. Yet guidelines for quality grading cultured pearls often vary with the mollusk species. For example, the acceptable nacre thickness for the export of black cultured pearls from Tahiti is 0.6 mm (scheduled to change to 0.8 mm on July 31, 2002; "Pearl thickness controls…", 2001). This would be exceptional for an Akoya cultured pearl (grown in the Pinctada fucata martensii), as it would require a culturing period of about

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