The Stiff Pen Shell and its Nacre
The nacreous shell layer of Pen Shells imparts the trademark, dark iridescence that is so typical to the inner surface of Pen and other shells. The nacreous layer, or mother-of-pearl, is composed of thousands of very thin, flat, colorless, translucent calcium carbonate crystals (also known as tablets). These flat crystals cause incident light to scatter and reflect back at different angles, which is perceived as distinctive colors. The tablets are separated thin layers of org