During diagenesis, construction material hardening, or biomeralization, the transformation of loose grains into a cohesive solid requires the crystallites to grow eventually constrained by the surrounding grains. Whereas it has never been measured, this confinement and the associated stress are expected to influence noticeably the growth and the final properties of the material. We report here on atomic force microscopy measurements of atomic step velocity during calcite growth, with a varying stress applied by the tip to the surface. The stress has a double influence: it both slows the growth and modifies the material crystalline phase. Furthermore, the addition of a small quantity of oligopeptide is shown to have no significant influence on the kinetics but to completely cancel the phase change under stress. Our results emphasize the previously unknown role of stress on growth mechanisms and identify a new possible role of organic molecules in tuning the morphology of biomineralized materials.