Possible Natural Abalone Shell Blister
An intriguing item was sent by KC Bell to GIA’s Carlsbad laboratory for examination. Mr. Bell stated that it was found among a group of natural abalone pearls off the coast of Southern California in the Pacific Ocean. The sample (figure 1) weighed 3.00 ct, measured 10.73 × 8.82 × 6.23 mm, and was appealing because “pearl” layers appeared to have formed around a piece of rock. However, its natural or cultured origin and the mollusk species that produced it needed to be determined.
The sample consisted of two main components: an opaque dark body and a translucent cap formed of nacreous layers. The main body looked like a stone with a typical angular baroque shape expected of unfashioned natural material. The exposed areas exhibited a bumpy surface texture, and a strong fiber-optic light revealed a dark greenish bodycolor and some black areas of a different composition. Subsequently, Raman spectroscopic analysis with an 830 nm diode laser verified the dark greenish body was composed of a rock-forming feldspar mineral when the spectrum was matched with the RRUFF mineral database (RRUFF-R040129).
The thin layers partially covering the stone displayed vibrant blue, green, and purple-pink hues. At higher magnifications, a fine platelet structure was clearly visible, together with a characteristic underlying botryoidal-like structure that has been observed through the translucent surface layers of many abalone pearls (Fall 2015 Lab Notes, pp. 319–320). Raman analysis of the surface layers using a 514 nm argon-ion laser revealed peaks at 701, 705, and 1085 cm–1, proving the nacre was composed of aragonite. In pearl testing, the higher excitation power of this laser is generally used in the identification of calcium carbonate polymorphs as well as natural pigments or artificial coloring agents. The 830 nm laser is used to obtain spectra in some situations, such as when high fluorescence is encountered with the 514 nm laser.
Energy-dispersive X-ray fluorescence (EDXRF) analysis showed that the nacreous surface layers contained low levels of manganese and confirmed a saltwater origin. Moreover, 221 ppm of iodine was detected, typical of many abalone shells and pearls since abalone is an iodine-rich organism (H.S. Doh and H.J. Park, “Speciation of bio‐available iodine in abalone (Haliotis discus hannai) by high‐performance liquid chromatography hyphenated with inductively coupled plasma‐mass spectrometry using an in vitro method,” Journal of Food Science, Vol. 83, No. 6, 2018, pp. 1579–1587). GIA has often detected iodine in abalone samples examined (Fall 2015 Lab Notes, pp. 319–320). Surface observation and chemical data confirmed that the nacre layers were produced by a univalve abalone gastropod (Haliotis genus), allowing us to confidently identify the mollusk species. Furthermore, EDXRF analysis of the rock surface revealed that it was composed of sodium-rich feldspar.
Closer examination of a small surface opening on the face of the sample (figure 2) and the edge of the nacre where it did not cover the underlying rock (figure 1, center and right) revealed a tight connection between the two main components. No gas bubbles or other features such as flow lines indicating an artificial bonding agent were observed in the translucent brown layer between them. The brown layer appeared to be an organic substance—likely conchiolin, an essential natural substance secreted by mollusks during the formation of pearls, blisters, and shells. The internal structure revealed by real-time X-ray microradiography (RTX) supported the observational findings that the thin nacreous overgrowth adhered to the rock’s surface in all directions and did not appear to have formed from any culturing process (figure 3).
Abalone are known to produce natural pearls and are also used commercially to produce cultured shell blisters in order to fashion assembled cultured blister “pearls” (mabe). Shell blisters can form naturally when a foreign object accidentally enters the mollusk’s body and becomes trapped in between the shell’s interior surface and the mantle tissue. If the mollusk survives this experience, the cells within the mantle tissue continue to secrete nacre (commonly biogenic aragonite) to increase the layers and cover the intruder, possibly to ease any discomfort. The intruder eventually becomes a part of the inner shell, resulting in a natural shell blister. Based on this formation mechanism and the gemological evidence obtained, this sample appears to be an unusual natural shell blister in which a small piece of rock acted as the irritant that initiated formation. Since abalone mollusks inhabit rocky areas and attach themselves to rocky surfaces, this scenario is not out of the question.
