Provided are compositions containing amorphous calcium carbonate (ACC), and at least one phosphorylated amino acid or phosphorylated peptide which stabilizes the amorphous form of said calcium carbonate. Particularly, the peptide can be selected from crustacean proteins, also provided by the invention, namely GAP65, GAP22, GAP21, and GAP12. The compositions are useful in pharmaceutical and nutraceutical formulations.