SEASONAL SHIFTS IN FRESHWATER PEARL MINERALOGY AND CHEMISTRY (Invited Presentation)

Across human history, pearls have been important gemstones worldwide. Today, they comprise a billion-dollar industry dominated by “cultured” pearls farmed by humans in both saltwater and freshwater environments. Yet, relatively little is known about how pearl nacre growth and quality will be impacted by changes in climate and local environments.

To better understand this, our study took a multi-disciplinary approach combining gem science knowledge with traditional mineralogical techniques (Raman spectroscopy, SEM imaging) and SIMS-based δ¹⁸O measurements to examine how environmental factors (i.e. temperature, dissolved oxygen) influence freshwater pearl mineralogy and chemistry on a sub-micron scale. We used three freshwater cultured pearls grown by the American Pearl Company between 1995 and 2004 in TN, USA as geochemical “time capsules” which represent precise time frames of nacre growth matched to environmental data from the nearby Kentucky Lake Long-Term Monitoring Program by Murray State University, KY, USA.

Our findings suggest that these freshwater pearls record variations in oxygen isotope ratios and crystallographic signatures over seasonal timescales. Specifically, we observe strong correlations between pearl δ¹⁸O values and lake temperature, dissolved oxygen, and light levels at 1m depth. The peak intensities of carbonate Raman vibrational modes are also zoned across the pearls on similar seasonal timescales and correlate with lake temperature and dissolved oxygen levels. Unexpectedly, nacre tablet thicknesses correlate with lake electrical conductivity, but not lake temperature.

We expect that this kind of detailed mineralogical approach to understanding how pearls grow under varying conditions will not only benefit the pearl farming industry and the economies of nations that depend on it, but may also help to inform environmental studies and paleoproxy studies on mollusks and other biomineralizing organisms.