X-ray luminescence, a valuable test in pearl identification

The increasing similarity of structures encountered in natural pearls and headless freshwater cultured pearls requires one or more additional criteria for their differentiation. The majority of natural pearls are from saltwater oysters; in contrast, most headless cultured pearls come from freshwater mussels. For some time it has been known that freshwater pearls produce luminescence under X-rays, whereas pearls grown in saltwater do not. The reason is because freshwater nacre contains traces of manganese. By using a sensitive camera this visible luminescence can be recorded and displayed on a monitor. The heads (from freshwater nacre) in Japanese saltwater cultured pearls (Akoya) also react to the X-ray excitation and may shine through the cultured overgrowths that are relatively thin. The method is used as an additional test and is not an alternative for X-radiograph images.