Grain size distribution is a fundamental property of polycrystalline materials and the phenomenon of grain boundary motion has major repercussions for materials processing and in a variety of technological applications. In classical models for microstructural evolution during grain boundary motion, special attention is given to the case of "ideal" grain growth defined by the seemingly unrealistic assumption of homogeneity of the physical properties of the boundaries. Whereas different models were rarely shown to demonstrate ideal microstructural evolution of selected synthetic and geological granular networks, here we present a biogenic polycrystalline material the formation of which is consistently described by all principal predictions of an ideal grain growth behaviour. We show that biomineralization of the prismatic architecture in the shell of the mollusc Atrina vexillum is fully quantified by conventional thermodynamic, kinetic and topological considerations, thus making a material formed by a living organism archetypical of ideal grain growth.