Research Papers

Black Non-Nacreous Natural Pearls from Pteria sp.

The Bahrain Institute for Pearls & Gemstones (DANAT), Manama, recently received a 5.70 ct black pearl (9.49–9.51 ´ 8.75 mm) and an 11.84 ct black and brown pearl (13.63–13.71 ´ 9.55 mm), both of button shape (Figure 1). Viewed with the microscope, the samples showed hexagonal-like cellular patterns linked with calcite columnar structures, similar to those observed on non-nacreous pearls of similar colour(Sturman et al., 2014). The brown part of the larger sample showed a nacreous appearance. EDXRF analysis revealed Sr/Mn>>12, characteristic of saltwater pearls.

Digital X-microradiographs of the samples in three orientations, taken perpendicular to one another, are shown in Figure 2. Lighter tones indicate materials with higher density 2 such as calcium carbonate, and darker tones represent lower-density materials such as organic matter or cracks. Both samples present radial structures, as well as concentric structures pronounced toward the rim and a dark centre (mainly observed in the larger sample; see middle and right radiographs at the bottom row of Figure 2) characteristic of natural pearls. The fully non-nacreous pearl also shows some cracks, mainly visible in the radiograph taken along the longest dimension (top-left in Figure 2). Small cracks also are visible in the centre of the other sample (bottom-left in Figure 2). It is worth noting that cracks in non-nacreous calcitic pearls are observed along their columnar structures.

Under long-wave UV radiation (365 nm, 6 watt), both samples exhibited orangey red fluorescence (Figure 3), similar to that observed in pearls from Pteria sp. (Kiefert et al.,2004). Under short-wave UV radiation (254 nm, 6 watt), both samples showed a very weak yellowish green reaction. A similar fluorescence reaction, which is linked with a kind of porphyrin, was observed for a partially non-nacreous and nacreous pearl from a Pteria penguin bivalve when viewed with the microscope using 300–410 nm excitation (Hainschwang et al., 2013). A porphyrin-type pigment also has been identified in natural and cultured pearls from other molluscs (e.g., Pinctadamargaritifera); however, samples from Pteria sp. present orange-red fluorescence to long-wave UV. Thus, even though black-coloured non-nacreous pearls are found in different molluscs (e.g., from the Pinnidae family, also known as pen shells commonly show chalky yellow fluorescence to long-wave UV; Sturman et al., 2014), the fluorescence of these two samples leads us to the conclusion that they originated from Pteria sp.Dr Stefanos Karampelas (Stefanos.Karampelas@danat.bh) and Hasan Abdulla Bahrain Institute for Pearls & Gemstones (DANAT), Manama, Bahrain