Microscopic Shells in Natural Pearls from Pinctada radiata
For centuries, natural pearl diving was Bahrain’s main industry. The vast majority of natural pearls fished around the islands of Bahrain are found in the Pinctada radiata bivalve (known in Arabic as mahar). These pearl beds have been known to produce the finest-quality natural pearls in the Arabian (Persian) Gulf region. Given the Gulf’s strategic trading location and the richness of its pearling industry, the pearls from this region are coveted by traders and collectors alike (R. Carter, “The history and prehistory of pearling in the Persian Gulf,” Journal of the Economic and Social History of the Orient, Vol. 48, No. 2, 2005, pp. 139–209). GIA’s Mumbai laboratory recently examined a quantity of these pearls obtained from two local Bahraini divers who claimed they were recovered from wild mollusks living in the nutrient-rich shallow waters off the coast of Sitra, one of Bahrain’s 33 islands.
Natural pearls from Pinctada radiata fished from Bahrain and the Gulf region in general have a wide range of shapes and colors. While most of the pearls examined showed a variety of interesting internal structures, two of them had a noteworthy feature: a minute shell contained within. Natural pearls are very rare and form inside mollusk shells, and a shell within a pearl within a shell is like nature’s version of a nesting doll. Such structures have been encountered by GIA on rare occasion (see Winter 2015 Lab Notes, pp. 434–436).
Both pearls were very small (figure 1). The smaller one (A) had a strong yellow bodycolor and an oval shape, weighing 0.06 ct and measuring 2.07 × 1.93 mm. The larger round one (B) had a light cream color, weighing 0.25 ct and measuring 3.30 mm in diameter. When viewed under 40× magnification, both exhibited typical nacreous overlapping aragonite platelets. Real-time microradiography (RTX) and X-ray computed microtomography (μ-CT) analyses were carried out to examine the internal structures in greater detail.
A minute gastropod shell measuring approximately 0.50 × 0.30 mm was observed in pearl A. The shell walls were very thin, and minimal growth arcs were present in the nacre surrounding it (figure 2). Marine gastropods are known to range in size from a few millimeters to more than a meter, so the size of the shell inside this pearl suggests it was from a juvenile gastropod just beginning to form its shell. The juvenile shell later forms the protoconch or first whorls of an adult gastropod (A. Nutzel, “Larval ecology and morphology in fossil gastropods,” Paleontology, Vol. 57, Part 3, 2014, pp. 479–503).
RTX imaging of pearl B revealed a minute shell that appeared to be a foraminifera test (shell), measuring approximately 0.45 × 0.35 mm. Foraminifera are small unicellular marine organisms found on the sea floor. A thin layer of organic matter that appeared darker in the RTX and μ-CT images seemed to envelop the foraminifera test, and a few growth arcs were observed within the surrounding nacre (figure 3). The μ-CT scan was also rendered using specialized software (C. Zhou et al., “New 3-D software expands GIA’s pearl identification capabilities,” GIA Research News, May 13, 2016) to create a three-dimensional image that made it easier to see the external morphology of the shell within the pearl (figure 4). The foraminifera test appeared multilocular, or multichambered, with tubular chambers arranged around a growth axis to form a beautiful milioline arrangement of chambers (A.R. Loeblich and H. Tappan, Foraminiferal Genera and Their Classification, Springer, New York, 1988). The minute size of the shells within these tiny pearls is a good indicator of their natural origin.
Energy-dispersive X-ray fluorescence spectrometry on pearls A and B revealed low manganese levels of 39.0 ppm and 17.8 ppm and high strontium levels of 1768 ppm and 1497 ppm, respectively, which is characteristic of formation in a saltwater environment. Raman analysis was also carried out using 514 nm laser excitation on the surface of each pearl. A doublet at 702 and 705 cm–1 as well as a peak at 1085 cm–1 indicative of aragonite were observed, along with minor polyenic pigment peaks at 1130 and 1530 cm–1. Photoluminescence (PL) spectra were also collected on both pearls. Pearl A revealed three broad peaks at 620, 650, and 680 nm, characteristic of many naturally colored pearls, while pearl B showed clear aragonite peaks and low fluorescence. An ultraviolet/visible reflectance spectrum was collected only for pearl B within the 220–850 nm range, as pearl A’s size prevented the detector from obtaining a clear result. Faint features at 420 and 495 nm were, like the PL results, consistent with natural coloration. Similar spectral observations have previously been documented in natural Pinctada radiata pearls (A. Al-Alawi et al., “Saltwater cultured pearls from Pinctada radiata in Abu Dhabi (United Arab Emirates),” Journal of Gemmology, Vol. 37, No. 2, 2020, pp. 164–179).
The gemological examination of these Pinctada radiata pearls proved very rewarding, especially with regard to their internal structures. Research into what causes the formation of a pearl in the wild is ongoing. Hence, finding these minute shells that may be the initiation of growth in these two natural pearls is a truly rare circumstance.
