The oldest structural colour there is, and the one this whole library is a model of. The animal lays down aragonite tablets a few hundred nanometres thick, sheet upon sheet with protein between them, and light reflecting from those layers interferes with itself. No shell contains a pigment that could make these colours. Because the tablets lie parallel to the surface rather than scattered through it, nacre glows in broad zones instead of glittering — and because the ribs turn the surface through a dozen angles at once, one glance holds the whole sweep.