Bio-inspired study on replicating the superior damage tolerance requires a detailed understanding of the intrinsic properties of biomineral units. Here, we investigate and compare the intrinsic properties of biogenic calcite (Atrina rigida) and aragonite (Sinanodonta woodiana) by conducting micro-bending experiments on the separated prismatic building blocks. Analyzed bending results indicate that the biogenic calcite has a higher modulus (36.24±14.4 GPa for A. rigida vs. 29.9±10.5 GPa for S. woodiana) and strength (446.5±141.5 MPa for A. rigida vs. 338.6±63.2 MPa for S. woodiana) than the biogenic aragonite, while the nanoindentation results indicate the opposite trend. Further systematic fractographic analysis suggests that the biogenic calcite fractures like amorphous glass, while the biogenic aragonite resembles polycrystalline ceramics. These contradictory behaviors of biogenic calcite and aragonite under tension- (micro-bending) and compression-dominated (nanoindentation) loading conditions are attributed to their different intrinsic structures, i.e., intracrystalline organic inclusions in single-crystal calcite vs. interlocked nanograins in polycrystalline aragonite.