The effects of Ag+, Al3+, and Cr3+ ions on calcium carbonate polymorphs were investigated using a double diffusion technique. The experimental results, analyzed by the equivalency rule, were as follows : (i) Ag+, Al3+, and Cr3+ ions promoted the formation of aragonite; (ii) in the presence of Ag+ ions, Ag2CO3 crystals acted as a nucleus for the epitaxial growth of aragonite; (iii) Al3+ and Cr3+ ions reduced the growth rate of the metastable phase (aragonite), which was formed at the initial stage of CaCO3 crystallization. Due to their adsorption onto growth sites, the supersaturation remained at a high level in this system. Aragonite was therefore able to be the dominant phase.