Dyed Non-Nacreous Pearls
Recently, 90 items purported to be pearls were submitted to GIA’s Bangkok laboratory for identification. While the majority were confirmed to be natural pearls, four samples warranted further study. Each of these four samples exhibited a non-nacreous surface, and on first impression they appeared to be conch pearls owing to their pink and light pink coloration. Their measurements ranged from 6.53 × 5.11 × 4.92 mm to 10.64 × 10.46 × 7.28 mm, and they weighed 1.30, 2.88, 4.41, and 5.88 ct, respectively (figure 1).
Observation through a 10× loupe and a microscope revealed that the samples were non-nacreous pearls. However, it was also readily apparent that the colors had been modified and were not natural. The 1.30 ct pearl exhibited the flame-like structure routinely observed in pearls such as conch, but the obvious red dye visible over the entire surface made it difficult to determine whether the flame structure was really present (figure 2A). The 2.88 ct pearl displayed a subtle flame structure, and some pink dye was visible on the surface and within many surface blemishes (figure 2B). Some surface areas of the 4.41 ct pearl showed the original bodycolor (figure 2C), but much of the surface showed the same dye features seen in the previous two pearls. Finally, the 5.88 ct pearl exhibited a nice flame pattern throughout, with some surface areas on the base and circumference modified by working (The Pearl Book, CIBJO Pearl Commission, 2020; N. Nilpetploy et al., “A study on improving the surface appearance of low-quality Pinctada maxima bead cultured pearls,” GIA Research News, March 24, 2021).
The color origin of this pearl was a little more challenging to identify because most areas were a lighter color, and as a result, the color modification evidence was harder to see with the unaided eye. However, magnification revealed pink dye concentrations within cracks on the base and in some surface-reaching structural features (figure 2D). Real-time microradiography (RTX) revealed a variety of structures within the pearls. The organic-rich and void features observed could be interpreted as characteristic of some non-bead cultured pearls. While they may be considered suspect, similar features have been observed in natural non-nacreous pearls (E. Fritsch and E.B. Misiorowski, “The history and gemology of Queen conch ‘pearls’,” Winter 1987 G&G, pp. 208–221; S. Singbamroong et al., “Observations on natural non-nacreous pearls reportedly from Tridacna (clam) species),” 34th International Gemmological Conference, Vilnius, Lithuania, 2015; Summer 2018 Lab Notes, pp. 211–212). To date, there have been no reports of any commercially produced non-nacreous cultured pearls in the market. Thus, based on surface observations and their internal structures, they were identified as treated-color natural non-nacreous pearls.
Further examination under long-wave ultraviolet radiation showed a weak to moderate red reaction over the colored areas, while the white area on the 4.41 ct pearl and the lighter-colored areas on the 5.88 ct pearl exhibited bluish reactions of moderate to strong intensity. Ultraviolet-visible (UV-Vis) reflectance spectra revealed absorption features expected for pink pearls in the visible range, but the absorption patterns differed from those typical of naturally colored conch pearls. Raman spectra collected using a 514 nm laser revealed peaks (doublet) related to the vibration modes of aragonite at 702 and 705 cm–1. All of the pearls showed additional weak peaks at 485, 640, and 827 cm–1 and less-defined features between 1000 and 1700 cm–1. However, none were associated with the polyenic peaks observed in most naturally colored non-nacreous pearls (Summer 2017 Lab Notes, pp. 230–231; Summer 2018 Lab Notes, pp. 211–212). The photoluminescence spectra also differed from those expected for naturally colored pink conch pearls and lacked the obvious polyenic peaks routinely observed in such pearls (figure 3).
Surface observation using the loupe and microscope combined with the results obtained from advanced analyses on the colored surface areas led us to conclude that the colors of the four pearls had been modified. While this is not so surprising when it comes to nacreous pearls, it is, from the author’s experience, more unusual to encounter color-modified natural non-nacreous pearls. Since these samples appeared visually similar to conch pearls, we can speculate that the original white or lightly colored pearls were dyed pink to imitate those produced by the Queen conch mollusk (Lobatus gigas, formerly known as Strombus gigas). This would make sense based on consumer demand in the market. However, the exact reason for treating these particular samples is unknown.
