Spiral Nacre in Natural Pearl
Nacreous pearls are known to have platelet growth resembling fingerprints or topographic maps, but a less common appearance is nacre with a concentric spiral pattern. The feature is sometimes seen in small patches in unprocessed saltwater pearls. One recent example was exceptional as it showed spiral formations throughout its entire surface (see above). The pearl was determined to be a natural saltwater pearl from the Pinctada genus. Nacre is composed of aragonite crystals and conchiolin protein, with the crystals acting like bricks and the protein acting like mortar. The explanation for spiral patterns is complex and involves the development of screw dislocation defects during nacre formation as predicted by the Burton-Caberra-Frank theory (J.H.E. Cartwright et al., “Spiral and target patterns in bivalve nacre manifest a natural excitable medium from layer growth of a biological liquid crystal,” PNAS, Vol. 106, No. 26, 2009, pp. 10499–10504).
