Chang, Y.; Tseng, H.-H.; Masahiko, T.; Pohl, D.; Rellinghaus, B.; Bertinetti, L.; Politi, Y.

Brilliant structural colours originating from diverse photonic crystals are found across many phyla, including the striking iridescent colours of beetles and butterflies, produced by three-dimensional photonic crystal structures in the specialized cuticular scales. However, the precise composition of these structures remains largely unknown, although it is key to unravelling colour production mechanisms and morphogenesis. The longhorn beetle Doliops similis displays vibrant green patterns on its otherwise dark elytra. These patterns are formed by arrays of minute scales that encompass a three-dimensional photonic crystal made of orderly packed nanospheres. We found that these nanospheres are composed of carbonated amorphous calcium phosphate biomineral. By accurately parameterizing the structure and calculating the refractive index deduced from the relative fractions of the organic and inorganic phases, we derived reflection wavelengths that match the observed green hue, demonstrating the role of biominerals in colour production. Our study further reveals that biomineralization is widespread in the Lamiinae subfamily, with colour diversity achieved through variations in nanosphere size, packing, and composition. This study opens new avenues for developing bioinspired mineral-based optical devices with high refractive indices and defect-resistance, overcoming the shortcomings of current polymer-based designs.