Nacre–the iridescent inner lining of many mollusk shells– has a unique strcuture that is remarkably resistant to fracture. The nacre featured on this cover is from Haliotis laevigata with average layer thickness 470-nm. The colors represent crystal orientationthe crystal lattice tilts across tablets.left stack all tablets are yellow, hence all aragonite crystals are co-oriented. The central stack shows a color gradient from red to black, thus,. In this PIC-map color represents crystal orientation Read more.