Biologically controlled minerals are often intimately associated with occluded organic components that regulate composition and morphology, and induce complex hierarchical structures. This integration of strong but brittle inorganic with tough but compliant organic also enables these hybrid tissues to have remarkable mechanical properties considering the starting materials. Unfortunately, this complexity also makes is difficult to isolate the individual effects of structure and composition on the mechanical response of the system. Here, we develop methods using synthetic mineralization to examine the effects of crystallographic orientation and additive content, on the hardness of single crystal calcite to gain insight into biomineralization. Using quasistatic depth sensing nanoindentation, we compare the hardness of synthetic calcite crystals incorporating magnesium, amino acid, or agarose additives to biogenic calcite from the prismatic layer of the mollusk Atrina rigida and geologic calcite in the form of Iceland spar. The hardness of single crystal calcite on the (001) face, varies with azimuthal angle; about 7% for Iceland spar and 20% for biogenic calcite. Additionally, this range in hardness increases with higher additive content. Hardness also increases by 30, 70 and 20% by adding magnesium, amino acids, and agarose polysaccharides, respectively, equaling the ~70% difference between biogenic and geologic calcite. Hardness can be reduced by thermal decomposition of the polysaccharide reinforcement, though the final hardness is still greater than pure geologic calcite. The variations in hardness with azimuthal angle and additive content are consistent with a hardening mechanism based on hindered dislocation motion.