Fascinating Snakeskin Pattern on a Pen Shell
The pen shell, a member of the Pinnidae family, is distinguished by its thin, fragile walls and fan-shaped outline. While typically ranging from 30 to 50 cm in length, the pen shell can reach sizes up to 120 cm, making it one of the largest endemic bivalve mollusks of the Mediterranean Sea. It is usually found in estuaries and coastal waters (T. Capello et al., “Pen shell Pinna nobilis L. (Mollusca: Bivalvia) from different peculiar environments: Adaptive mechanisms of osmoregulation and neurotransmission,” European Zoological Journal, Vol. 86, No. 1, 2019, pp. 333–342).
Pen shells are known for their intricate pattern of concentric growth lines, which form a visually captivating display, and their coloration ranging from brown to vibrant hues of yellow. GIA’s Mumbai laboratory occasionally receives shells with intriguing features, one of them a pen shell with broken edges from Bahrain, measuring approximately 11.9 × 7.4 cm (figure 1). When viewed under magnification with fiber-optic illumination, a small area revealed an organized translucent yellow to brown cellular pattern, with vivid yellow outlines around each cell. These resembled a scale pattern, akin to the skin of a snake. Interestingly, a thick, milky white material filled the boundaries between the cells, giving the shell a shiny appearance. It was evident that the shell had been coated with varnish to enhance its aesthetic appeal (figure 2).
While shells can exhibit many different appearances, this snakeskin pattern is particularly fascinating.
