SHELL PHOTONIC CRYSTALS. “The dry ligament of Pinctada maxima normally appears black; however, it can exhibit striking blue structural colors after being wetted by water. The field-mission SEM investigation shows that the ligament is made of lamellae … the fibers are highly aligned characterized by a 2D photonic crystal type structure.”
http://link.springer.com/article/10.1007/s11434-007-0142-z