Natural Non-Nacreous Pearls from Various Marine Mollusks
GIA’s Bangkok laboratory received for identification a set of 35 loose non-nacreous pearls displaying a wide range of bodycolors including white, orange, pink, brown, purple, and black. The pearls were near-round to round in shape and exhibited surface structure characteristics from various marine mollusk species (figure 1). Arranged in a necklace layout, they weighed from 3.96 ct to 51.63 ct and measured 8.40 × 8.01 mm to 19.20 × 18.93 mm (figure 2).
Observation with a 10× loupe and a microscope showed no evidence of color treatment on any of the pearls. Raman spectroscopy with a 514 nm argon-ion laser, as well as an 830 nm diode laser, was used to confirm color origin and examine the surface composition of all the pearls. The majority of the colored pearls displayed two main natural polyenic pigment peaks at 1120–1135 cm–1 and 1505–1530 cm–1, indicating their natural color origin. Real-time X-ray microradiography and energy-dispersive X-ray fluorescence analyses confirmed these were natural saltwater pearls.
Species determination was based mainly on the combination of colors and external appearances, internal growth structures, spectroscopic properties, and surface characteristics under magnification. Most of the pearls were porcelaneous and exhibited various flame structure patterns and high porcelain-like luster. All the white pearls were typical clam pearls produced from the Tridacninae subfamily. Under microscopic examination, many of the clam pearls displayed well-arranged flame patterns radiating from the centers (J-P. Gauthier et al., “Evidence of rotation in flame-structure pearls from bivalves of the Tridacnidae family,” Summer 2019 G&G, pp. 216–228). The Melo pearls (from Melo species) were a yellowish orange color with visible flames that appeared to have jagged edges (Summer 2021 Lab Notes, pp. 152–153). Cassis pearls (from Cassis species) were present in various hues and saturations of light orange, pinkish orange, and orangy brown. They shared characteristic long slender flames, some of which were overlapping (figure 3A). Iridescent concentric rings (Newton’s rings) were also observed slightly below the surface of some Cassis pearls (figure 3B), similar to a previously reported feature (Fall 2012 Lab Notes, pp. 211–212). Another orangy brown Cassis pearl had a non-nacreous surface with a mottled appearance containing irregular streaks and patches but lacked any flame pattern (figure 3C; C. Zhou et al. “Disordered dolomite as an unusual biomineralization product found in the center of a natural Cassis pearl,” PLOS One, Vol. 18, No. 4, 2023, article no. e0284295). Porcelaneous pearls from the Fasciolariinae subfamily (horse conch), Aliger gigas (conch; formerly known as Strombus gigas and Lobatus gigas), and Spondylus species (thorny oyster) mollusk were also identified.
A group of scallop pearls from the Pectinidae family were identified in the collection. These pearls exhibited non-nacreous surfaces in the form of calcite, displaying a segmented patchwork of cells commonly found in scallop pearls and shells (figure 4, left). Some showed brownish purple and purplish brown colors typically produced by Nodipecten species, also known as the lion’s paw scallop (K. Scarratt and H.A. Hänni, “Pearls from the lion’s paw scallop,” Journal of Gemmology, Vol. 29, No. 4, 2004, pp. 193–203). An additional black calcite pearl was concluded to be a pen pearl from the Pinna species. This pearl was semitranslucent and exhibited characteristic cellular surface structure (figure 4, right; N. Sturman et al., “Observations on pearls reportedly from the Pinnidae family (pen pearls),” Fall 2014 G&G, pp. 202–215; Winter 2014 Gem News International, pp. 305–306).
Finally, a few pearls in the collection exhibited borderline or unusual characteristics, and thus the producing mollusk species could not be determined. These included an orange pearl with a spiral surface pattern (Spring 2025 G&G Micro-World, pp. 77–78).
These pearls were likely produced in oceans and seas around the world, from the South China Sea to the Gulf of California (Sea of Cortez) and the Caribbean Sea. Finding a good-quality saltwater natural non-nacreous pearl is challenging, and the creation of this necklace layout required a great deal of effort and years of perseverance. The authors were grateful for the opportunity to examine this extraordinary set of pearls from a wide range of marine mollusk species.
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