A graduated strand of what appeared to be “golden” South Sea cultured pearls arrived at the GIA Gem Laboratory in Carlsbad for identification (see figure). The Laboratory routinely examines these types of cultured pearls to determine whether or not they are dyed. This strand, however, provided another unexpected result.

Magnification revealed that the string knots between the cultured pearls were yellow, while the rest of the string was white. There were also concentrations of darker yellow around some of the drill holes. These preliminary observations suggested that the color was artificial in origin. The reaction to long-wave UV radiation was also characteristic of treated-color pearls: Some fluoresced medium orangy yellow (occasionally mottled with blue), while others were medium pink with uneven orange mottling. Naturally and uniformly colored yellow pearls evenly fluoresce yellow to greenish yellow or greenish brown to brown; pink and orange components, along with blue patches and other mottling, are inconsistent with an evenly distributed natural color.

The UV-Vis reflectance spectra were characterized by Research Gemologist Shane Elen. The spectra consistently revealed a strong absorption trough in the blue region at approximately 415–440 nm rather than a deeper trough in the UV region at approximately 330–385 nm. These results are consistent with dyed yellow saltwater cultured pearls (see S. Elen, “Spectral reflectance and fluorescence characteristics of natural-color and heat-treated ‘golden’ South Sea cultured pearls,” Summer 2001 Gems & Gemology, pp. 114–123). As would be expected with bead-nucleated pearls, the X-ray fluorescence was weak to strong.

Treated-color yellow South Sea cultured pearls have become commonplace. The surprise with this necklace came while examining the X-radiographs. Rather than the expected bead nuclei, the radiographs revealed the characteristic structure of tissue-nucleated cultured pearls, immediately raising suspicion of freshwater origin.

EDXRF analysis (also performed by Elen) confirmed a level of manganese (Mn) that was consistent with a Texas freshwater pearl in the reference file. Mn can be found as a trace element in some saltwater pearls, but the amount present in these cultured pearls excluded the possibility of saltwater origin, thus proving they were freshwater cultured pearls that had been dyed to resemble “golden” South Sea cultured pearls. Although we do not have comparative UV-Vis spectra for dyed or natural-color yellow freshwater cultured pearls, the concentrations of color around the drill holes and unusual fluorescence behavior proved that these samples were dyed.

Cheryl Y. Wentzell
GIA Gem Laboratory
Carlsbad, California