Calcium carbonate is the most abundant biomineral that is biogenically formed with a vast array of nano and microscale features. Among the less stable polymorphs present in mineralized organisms, the most soluble, amorphous calcium carbonate (ACC), formed in chitin exoskeletons of some crustacea, is of particular interest since aqueous stability of isolated ACC is limited to a few hours in the absence of polyanions or magnesium. Here the influence of a selection of biopolymer gels on the mineralization of calcium carbonate is investigated. Mineralization is achieved in all biopolymers tested, but is particularly abundant in collagen hydrogels, in which a significant proportion of the calcium carbonate (≈18%) is found to be amorphous. In dense collagen gels, this amorphous fraction does not crystallize for up to six weeks in deionized water at room temperature. The reason why collagen in particular should stabilize this phase remains obscure, although the results suggest that the fiber diameter, fiber spacing, and the amphoteric nature of collagen fibers are important. Upon immersion in phosphate containing solutions, the calcium carbonate present within the collagen hydrogels is readily converted to carbonated hydroxyapatite, enabling the formation of a stiff bone-like composite containing 78 wt% mineral, essentially equivalent to cortical bone.