Amorphous calcium carbonate (ACC) can readily be prepared using ethanol as the reaction medium and ammonium carbonate as the source of carbon dioxide. Other additives, or any elaborate pH control are not needed to form the initial calcium carbonate precipitate. Ammonia generated from ammonium carbonate maintains the reaction medium in a neutral or weakly basic condition, retarding the crystallization of ACC, while ethanol itself inhibits the dissolution of ACC. The ACC prepared in this way provides a rare opportunity to fabricate molded biomimetic crystals in vitro, but the ACC is too fragile to be fabricated into proper shapes. The malleability of ACC is, however, greatly enhanced by incorporating poly(ethylenimine) (PEI). The ACC/PEI composite can then be fabricated, using a proper mold or template, into mechanically durable biomimetic crystals of definite shape. The ACC in the ACC/PEI composite can further be transformed into vaterite by heating under N2 atmosphere, while the native ACC simply converts into calcite.