Lab Notes Gems & Gemology, Winter 2022, Vol. 58, No. 4

Calcite Found on the Surface of a Saltwater “Nacreous” Pearl

Cheryl Ying Wai Au

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Figure 1. A cream-colored button-shaped loose pearl weighing 2.32 ct, submitted for identification. Photo by Tony Leung.
 
 
Figure 1. A cream-colored button-shaped loose pearl weighing 2.32 ct, submitted for identification. Photo by Tony Leung.

The Hong Kong laboratory recently examined a loose pearl weighing 2.32 ct and measuring 8.45 × 7.27 × 5.36 mm (figure 1). Externally, the cream-colored pearl did not look out of the ordinary and exhibited a soft luster with a relatively clean surface. Examination through a binocular microscope revealed typical overlapping platelets, pits, and scratches. No coating, surface working, or other indications of treatment were observed (figure 2).

Figure 2. The pearl exhibited overlapping surface platelets typically found in nacreous pearls. Photomicrograph by Nick Chan; field of view 1.10 mm.
 
 
Figure 2. The pearl exhibited overlapping surface platelets typically found in nacreous pearls. Photomicrograph by Nick Chan; field of view 1.10 mm.
Figure 3. RTX examination revealed a clear boundary running across the pearl as well as structures associated with natural formation. Images by Cheryl Ying Wai Au.
 
 
Figure 3. RTX examination revealed a clear boundary running across the pearl as well as structures associated with natural formation. Images by Cheryl Ying Wai Au.

Real-time microradiography (RTX) revealed a distinct boundary with twin growth structures consisting of normal growth layers that followed the shape of the pearl (figure 3). No structural characteristics of any known type of cultured pearl were observed, so the pearl was identified as a natural pearl. Energy-dispersive X-ray fluorescence analysis showed high levels of strontium and no traces of manganese. This, together with its lack of visible fluorescence to X-ray luminescence, confirmed its saltwater origin.

Figure 4. Raman spectra of three spots on the pearl’s surface exhibit main peaks at 156, 282, 712, and 1087 cm<sup>–1</sup> that are diagnostic of calcite.
 
 
Figure 4. Raman spectra of three spots on the pearl’s surface exhibit main peaks at 156, 282, 712, and 1087 cm–1 that are diagnostic of calcite.

Despite the typical saltwater nacreous pearl surface appearance, Raman analysis on three random spots with 514 nm laser excitation revealed that the pearl’s platelet surface was composed of calcite rather than aragonite, with peaks at 156, 282, 712, 1087, and 1437 cm–1 (figure 4) (J. Urmos et al., “Characterization of some biogenic carbonates with Raman spectroscopy,” American Mineralogist, Vol. 76, 1991, pp. 641–646).

Nacreous pearls are typically composed of small polygonal tablets of aragonite arranged in laminar layers (L. Addadi and S. Weiner, “A pavement of pearl,” Nature, Vol. 389, 1997, pp. 912–915). Up until the examination of this specimen, Raman analysis on submitted pearls and research samples exhibiting overlapping platelet layers has always shown a mineral composition of aragonite. Calcite is usually found in the form of mosaic or cellular patterns on the surface of some types of pearls, or in the prismatic internal layers. This is the first time GIA has encountered a “nacreous-looking” pearl showing calcite when tested in different positions on its surface, which is unique and certainly worthy of note.

Cheryl Ying Wai Au is a senior staff gemologist at GIA in Hong Kong.