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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ures 1 and 2). These objects have been described previously with regard to their historical and cultural importance (Burckhardt, 1933;Barth, 1990), but not with regard to the materials themselves.Only rarely does a gemologist get the opportunity to examine the stones adorning historical religious items. The investigations of Meixner (1952), CISGEM (1986), Köseoglu (1987), Querré et al. (1996), Bouquillon et al. (1995), Querré et al. (1995), and Scarratt (1998) are notable exceptions. The prudence and concern of museum curators is undoubtedly a major reason why so many important historical and religious pieces of jewelry and other artwork lack precise descriptions of their materials. Another explanation is that the fancy names and historical terminology for gemstones in old inventories are often simply accepted, without taking into account

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