  Biomineralizing organisms are widely noted and extensively studied due to
their ability to generate structures exhibiting exceptional crystallographic
control. Primarily, it is the organisms, such as sea-urchins or bivalves, that
generate nearly single-crystalline biocrystals that have attracted attention.
In contrast, biomineralizing organisms with seemingly disordered
polycrystalline bio-armor have been left relatively unstudied. However, the
crystalline ordering in the black-lipped pearl oyster, Pinctada margaritifera,
reveals that biominerals with varying crystal textures are an unrecognized
class of functionally graded materials. Changing crystal textures inevitably
cause a variation in Young modulus due to the orientation-dependent mechanical
properties of crystals. The case of Pinctada margaritifera demonstrates that
bioceramics with such crystallographical gradients are toughened by stress
delocalization and reduced stress intensity factors, outperforming non-graded
counterparts. These findings suggest that a multitude of biominerals, which are
perceived as poorly ordered because of their polycrystallinity and changing
crystal textures, may be considered as graded materials with hitherto
unidentified emergent mechanical properties. The underlying design principle is
remarkably simple and applicable to a wide range of crystalline material
classes and may thus serve as a blueprint for future bioinspired functional
materials.
