The shapes of biologically formed calcite crystals are extremely versatile. Numerous studies have addressed the possible biological mechanism of crystal shape regulation. Synthetic assays have shown that the shape and morphology of calcite crystals can be modulated by inorganic or organic solution additives. Hardly any studies have to date discussed the concept of controlling the shape of these crystals by organic nucleating surfaces. We show in this paper that self-assembled monolayers (SAMs) that template calcite nucleation have two pronounced effects: in addition to inducing the highly oriented crystal growth (the phenomenon that we have extensively described in our previous studies), each SAM induces a clear modification of the calcite shape from its equilibrium rhombohedron. We demonstrate that this change in shape originates from the anisotropy of lattice mismatches that develop between the nucleating crystal face and the organic SAM in different directions. We present a model that gives qualitative predictions for the shape of crystals grown on a variety of SAM substrates, as a function of lattice mismatch, and show that these shapes correlate extremely well with the experimental results. We believe that this mechanism might be utilized by organisms in the biomineralization process.