Akoya pearl farming was established in Japan more than a century ago and subsequently expanded to other countries such as China, Vietnam, and Australia. The akoya pearl oyster includes members of mollusks that belong to the Pinctada fucata, martensii, radiata, and imbricata species complex (K.T. Wada and I. Tëmkin, “Taxonomy and phylogeny,” in P.C. Southgate and J.S. Lucas, Eds., The Pearl Oyster, Elsevier, Oxford, 2008, pp. 37–75). The P. fucata (martensii) is the most widely used species to produce akoya cultured pearls.
Akoya bead cultured (BC) pearls are characterized by their round shapes and high luster. The growth period typically ranges from 10 to 14 months, which is shorter than that of South Sea (P. maxima) and Tahitian (P. margaritifera) BC pearls. A longer growth time can possibly impact the roundness of the pearls. Akoya mollusks can produce high-quality nacre when the water temperature is between 15° to 20°C, as cool water slows the rate of nacre deposition. Due to the shorter growth period and lower temperature growth conditions, akoya BC pearls are generally known to have a thinner nacre overgrowth than South Sea and Tahitian BC pearls. This also implies that a uniform white shell bead with no discoloration must be used during the nucleation process since low-quality beads can affect the quality of the final pearls (N. Sturman et al., “Vietnam: Shell nuclei, pearl hatcheries, and pearl farming,” Fall 2020 G&G, pp. 402–415). Very thin nacre can impact the durability and luster of pearls. As a result, many pearl farmers continually refine their techniques to achieve the highest quality with respect to shape, luster, and nacre thickness on their final products while balancing culturing time and cost.
GIA’s Bangkok laboratory received 91 hybrid akoya bead cultured pearls for study, weighing a total of 53.54 carats and ranging from 4.00 × 3.93 mm to 6.44 × 6.28 mm in diameter, from Orient Pearl (Bangkok) Ltd (figure 1). Reportedly produced at a pearl farm in Nha Trang, Vietnam, the pearls were harvested in April 2023 from hybrid mollusks that were crossbred between P. fucata (martensii) and P. radiata species. These akoya pearls are said to have a thicker nacre than typical akoya BC pearls produced from pure P. fucata (martensii) species, as the hybrid mollusks are more tolerant of warmer water, and therefore the nacre of their pearls can grow faster. However, only a very small percentage (around 0.5%) of the total production of these hybrid mollusks yields pearls with nacre significantly thicker than those from pure P. fucata mollusks. Traditional American freshwater shell beads were used to grow the pearls, with a cultivation period of only 5 months for the 4 mm pearls and about a year for the 6 mm pearls.
Most of the samples (87 out of 91) were about 4 mm in diameter, and the other four samples were 6 mm. All pearls were round with predominantly cream color, most having noticeable orient and pink and green overtone. They all had fine nacre, with no evidence of processing or color enhancement on their surfaces. The pearls fluoresced a very weak to moderate yellowish green under long-wave UV light. Short-wave UV fluorescence spectroscopy with 275 nm LED excitation resulted in counts of high intensity for these pearls, indicating that they were naturally colored and lacked the processing routinely applied for akoya pearls (Spring 2020 Lab Notes, p. 136). Ultraviolet/visible spectroscopy revealed absorption features around 435 and 460 nm in some of the pearls, which are characteristic features reportedly found on P. 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).
Real-time X-ray microradiography (RTX) analysis revealed that each pearl showed a thick nacre layer with a round bead nucleus in the center. Some of the beads were surrounded by an organic-rich material, as evidenced by dark gray areas with RTX. X-ray radiography was used to obtain the measurements. The four 6 mm pearls had a nacre thickness between 1.62 to 1.68 mm (figure 2, left), significantly thicker than the typical nacre range of 0.15 to 0.50 mm in traditional akoya pearls (figure 2, right). Nacre thickness of the 4 mm pearls ranged from 1.11 to 1.46 mm.
The degree of X-ray fluorescence (XRF) reaction observed in saltwater BC pearls is dependent on nacre thickness. Freshwater shell beads exhibit a high intensity of greenish yellow fluorescence due to trace amounts of manganese, while nacre produced in saltwater is generally inert. Thin nacre often allows a bead’s fluorescence to pass through, resulting in the pearl displaying noticeable fluorescence. Due to their thick nacre, the four hybrid akoya pearls either exhibited weak fluorescence or were inert to XRF.
Additionally, the diameters of the pearl (D) and bead (d) can be used to calculate the ratios of both nacre volume and bead volume to total pearl volume, as illustrated in table 1. From the calculation, all four 6 mm pearls have a high percent nacre volume of around 90% of the total pearl volume. Therefore, the bead nucleus only occupies around 10% of the total pearl volume. The nacre volume of all 4 mm pearls ranged from 90–94% of the total pearl volume.
For additional study, two of the 6 mm pearls were selected for X-ray computed microtomography (μ-CT) analysis. Specialized software to render the μ-CT scan images was used to generate three-dimensional models and measure nacre thickness and volume percentage of the pearl nacre and the bead. The results obtained were consistent with the calculation results from RTX analyses (table 2).
This study demonstrated that the hybrid mollusk species of P. fucata (martensii) and P. radiata can produce BC pearls with a thick nacre layer, with the nacre accounting for approximately 90% of the total pearl volume. Furthermore, according to the GIA 7 Pearl Value Factors classification system, the hybrid pearls studied displayed Excellent and Very Good lusters. Therefore, the thick and fine quality of the nacre resulted in high luster. A sufficient nacre layer can enhance durability, which is an essential characteristic that contributes to the value and beauty of akoya BC pearls.
Akoya pearl farming was established in Japan more than a century ago and subsequently expanded to other countries such as China, Vietnam, and Australia. The akoya pearl oyster includes members of mollusks that belong to the Pinctada fucata, martensii, radiata, and imbricata species complex (K.T. Wada and I. Tëmkin, “Taxonomy and phylogeny,” in P.C. Southgate and J.S. Lucas, Eds., The Pearl Oyster, Elsevier, Oxford, 2008, pp. 37–75). The P. fucata (martensii) is the most widely used species to produce akoya cultured pearls.
Akoya bead cultured (BC) pearls are characterized by their round shapes and high luster. The growth period typically ranges from 10 to 14 months, which is shorter than that of South Sea (P. maxima) and Tahitian (P. margaritifera) BC pearls. A longer growth time can possibly impact the roundness of the pearls. Akoya mollusks can produce high-quality nacre when the water temperature is between 15° to 20°C, as cool water slows the rate of nacre deposition. Due to the shorter growth period and lower temperature growth conditions, akoya BC pearls are generally known to have a thinner nacre overgrowth than South Sea and Tahitian BC pearls. This also implies that a uniform white shell bead with no discoloration must be used during the nucleation process since low-quality beads can affect the quality of the final pearls (N. Sturman et al., “Vietnam: Shell nuclei, pearl hatcheries, and pearl farming,” Fall 2020 G&G, pp. 402–415). Very thin nacre can impact the durability and luster of pearls. As a result, many pearl farmers continually refine their techniques to achieve the highest quality with respect to shape, luster, and nacre thickness on their final products while balancing culturing time and cost.
GIA’s Bangkok laboratory received 91 hybrid akoya bead cultured pearls for study, weighing a total of 53.54 carats and ranging from 4.00 × 3.93 mm to 6.44 × 6.28 mm in diameter, from Orient Pearl (Bangkok) Ltd (figure 1). Reportedly produced at a pearl farm in Nha Trang, Vietnam, the pearls were harvested in April 2023 from hybrid mollusks that were crossbred between P. fucata (martensii) and P. radiata species. These akoya pearls are said to have a thicker nacre than typical akoya BC pearls produced from pure P. fucata (martensii) species, as the hybrid mollusks are more tolerant of warmer water, and therefore the nacre of their pearls can grow faster. However, only a very small percentage (around 0.5%) of the total production of these hybrid mollusks yields pearls with nacre significantly thicker than those from pure P. fucata mollusks. Traditional American freshwater shell beads were used to grow the pearls, with a cultivation period of only 5 months for the 4 mm pearls and about a year for the 6 mm pearls.
Most of the samples (87 out of 91) were about 4 mm in diameter, and the other four samples were 6 mm. All pearls were round with predominantly cream color, most having noticeable orient and pink and green overtone. They all had fine nacre, with no evidence of processing or color enhancement on their surfaces. The pearls fluoresced a very weak to moderate yellowish green under long-wave UV light. Short-wave UV fluorescence spectroscopy with 275 nm LED excitation resulted in counts of high intensity for these pearls, indicating that they were naturally colored and lacked the processing routinely applied for akoya pearls (Spring 2020 Lab Notes, p. 136). Ultraviolet/visible spectroscopy revealed absorption features around 435 and 460 nm in some of the pearls, which are characteristic features reportedly found on P. 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).
Real-time X-ray microradiography (RTX) analysis revealed that each pearl showed a thick nacre layer with a round bead nucleus in the center. Some of the beads were surrounded by an organic-rich material, as evidenced by dark gray areas with RTX. X-ray radiography was used to obtain the measurements. The four 6 mm pearls had a nacre thickness between 1.62 to 1.68 mm (figure 2, left), significantly thicker than the typical nacre range of 0.15 to 0.50 mm in traditional akoya pearls (figure 2, right). Nacre thickness of the 4 mm pearls ranged from 1.11 to 1.46 mm.
The degree of X-ray fluorescence (XRF) reaction observed in saltwater BC pearls is dependent on nacre thickness. Freshwater shell beads exhibit a high intensity of greenish yellow fluorescence due to trace amounts of manganese, while nacre produced in saltwater is generally inert. Thin nacre often allows a bead’s fluorescence to pass through, resulting in the pearl displaying noticeable fluorescence. Due to their thick nacre, the four hybrid akoya pearls either exhibited weak fluorescence or were inert to XRF.
Additionally, the diameters of the pearl (D) and bead (d) can be used to calculate the ratios of both nacre volume and bead volume to total pearl volume, as illustrated in table 1. From the calculation, all four 6 mm pearls have a high percent nacre volume of around 90% of the total pearl volume. Therefore, the bead nucleus only occupies around 10% of the total pearl volume. The nacre volume of all 4 mm pearls ranged from 90–94% of the total pearl volume.
For additional study, two of the 6 mm pearls were selected for X-ray computed microtomography (μ-CT) analysis. Specialized software to render the μ-CT scan images was used to generate three-dimensional models and measure nacre thickness and volume percentage of the pearl nacre and the bead. The results obtained were consistent with the calculation results from RTX analyses (table 2).
This study demonstrated that the hybrid mollusk species of P. fucata (martensii) and P. radiata can produce BC pearls with a thick nacre layer, with the nacre accounting for approximately 90% of the total pearl volume. Furthermore, according to the GIA 7 Pearl Value Factors classification system, the hybrid pearls studied displayed Excellent and Very Good lusters. Therefore, the thick and fine quality of the nacre resulted in high luster. A sufficient nacre layer can enhance durability, which is an essential characteristic that contributes to the value and beauty of akoya BC pearls.