Lab Notes Gems & Gemology, Summer 2017, Vol. 53, No. 2

Natural Conch “Rosebud” Pearls

Pink “rosebud” conch pearls.
Figure 1. “Rosebud” conch pearls of various pink tones ranging from 0.93 to 14.72 ct. Photo by Sood Oil (Judy) Chia.

Non-nacreous conch pearls from Strombus gigas are known mainly for their very attractive pink color and distinctive shimmering appeal due to the characteristic flame structures on their surfaces. Although conch pearls are found in various tones of yellow, brown, and white, the most desirable hue is undoubtedly pink with an intense saturation. The flame structures are due to “crosswise arrays of bundles of aragonite laths or fibers that may reflect or absorb the light that falls on the structure, letting it appear bright or dull” (H.A. Hänni, “Explaining the flame structure of non-nacreous pearls,” The Australian Gemmologist, Vol. 24, No. 4, 2010, pp. 85–88).

In late 2016, GIA’s New York lab staff had the opportunity to study part of Susan Hendrickson’s conch pearl collection. Among the assortment of conch pearls of various sizes, colors, and shapes, a few “rosebud” specimens caught our attention (figure 1). The term “rosebud” is most commonly used to describe characteristically shaped freshwater nacreous pearls, but this is the first time GIA has examined conch pearls with such shapes (figure 2). The nine pink specimens had roundish outlines with a button-like appearance. But in keeping with the rosebud pearl form, they exhibited differences in the bumps or ridges on their surfaces. Some bumps were rounded and spread out, while others were jagged and tightly grouped. The authors could not locate any reports of rosebud conch pearls in the literature, and the cause of these distinctive surface structures is unknown. One possible explanation is that when a pearl forms in a pearl sac positioned in a region of muscular activity rather than the mantle, it will not form in a smooth symmetrical shape (E. Fritsch and E.B. Misiorowski, “The history and gemology of Queen conch ‘pearls’,” Winter 1987 G&G, pp. 208–221).

Shape comparison of American freshwater pearl and conch pearls.
Figure 2. A shape comparison of a 4.90 ct American freshwater pearl exhibiting rosebud form (A) and three conch pearls, weighing 1.30, 0.66, and 0.91 ct (B–D). Photos by Tino Hammid (A) and Joyce WingYan Ho (B–D).

All nine samples displayed typical flame structures under magnification (figure 3). Some of them did not exhibit the flames on the bumpy surface, but only within the smoother areas. Microradiography revealed a tight internal structure with uneven outlines corresponding to the bumpy surfaces. The Raman spectra were characteristic of aragonite and clearly showed additional natural polyenic pigment peaks, both as expected for pearls formed in Strombus gigas mollusks.

Characteristic flame structure.
Figure 3. This 6.21 ct rosebud conch pearl shows characteristic flame structure at the surface. Photos by Joyce WingYan Ho; fields of view 10.79 mm and 1.26 mm (inset).

There are records of attempts to cultivate conch pearls (H. Acosta-Salmón and M. Davis, “Inducing relaxation in the queen conch Strombus gigas (L.) for cultured pearl production,” Aquaculture, Vol. 262, No. 1, 2007, pp. 73–77; N. Sturman et al., “Cultured Queen conch pearls—A comparison to natural Queen conch pearls,” 32nd International Gemmological Conference, Interlaken, Switzerland, 2011; Summer 2015 GNI, pp. 201–202). No further news about the commercial production of cultured conch pearls has reached the market, however. As a result, conch pearls are still highly desirable and valued by the trade and specialty collectors. The opportunity to study such unique and exciting examples of rosebud conch pearls allowed GIA to expand its understanding of rosebud conch pearls for future reference.

Joyce WingYan Ho and Emiko Yazawa are staff gemologists at GIA in New York.