This article investigates the formation of nanostructured single crystals of calcite using direct, ion-by-ion precipitation methods and shows that single crystals with complex morphologies and curved surfaces can readily be formed using this technique. Calcite crystals with inverse opal and direct opal structures are prepared using templates of colloidal crystals and polystyrene reverse opals, respectively, and excellent replication of the template structures are achieved, including the formation of 200-nm spheres of calcite in the direct opal structure. These highly porous crystals also display extremely regular, crystalline gross morphologies. The methodology is extremely versatile and challenges the preconception that nanostructured crystals cannot be prepared by simple diffusion of reagents into the template due to blocking of the channels. The results are also discussed in light of alternative templating methods using amorphous calcium carbonate (ACC) as a precursor phase and provide insight into the role of ACC in biological calcification processes.