The function of biomacromolecules during biomineralization was studied by simulating the nucleation and growth of calcium carbonates in vitro. The synthetic crystals nucleated and grew on different organic (EDTA-insoluble proteins) and inorganic (silicon) matrices with EDTA-soluble proteins extracted from mollusk shells. The polymorph and morphology of the crystals were investigated using scanning electron microscope (SEM), X-ray diffraction (XRD) and transmission electron microscope (TEM) with selected area electron diffraction (SAED). It is found that different proteins have specialties to induct different crystals with special morphologies. The soluble proteins extracted from nacre can induce aragonite and the one from prism can induce calcite grown with a preferred orientation of [1 0 4]. The crystals induced by the same soluble proteins have the same polymorph and morphology. The insoluble proteins influence the density of nucleation sites as well as the sizes and quantities of the crystals.