Biominerals are biogenically generated ceramic materials which underwent evolutionary optimization over hundreds of millions of years under considerable selection pressure. This lead to high-performance materials with remarkably high functional density and, thus, light-weight characteristics and an intricate hierarchical design. Today, biominerals thus serve as an unfathomable source of design motifs valuable for new and effective design of functional ceramic materials. One of those distinct elements of design often employed in biominerals is a materials gradient which roots, e.g., in a variation in composition, particle size or even crystallinity. Investigating a shell of a calcareous, nacroprismatic bivalve, we reveal a new class of functionally graded material which is based on an exceptional control of crystallographic features during shell growth. The emerging gradient enhances significantly the damage and stress tolerance of the bivalve shell. This design concept does not find, yet, any counterpart in synthetic materials and is thus hitherto unparalleled by mankind. This unforeseen class of functionally graded materials changes our view on the properties and complexity of crystalline biominerals.