Using amorphous CaCO3 (ACC) to biomimic the crustacean exoskeleton and optimize the physical and chemical properties of the polymeric phase of ACC holds great promise. Controlling the ACC morphology and stability is key in this process. For this article, monodisperse ACC microspheres, with a high sphericity of 0.973 ± 0.001 and a hardness of 0.6755 GPa, were prepared using the gas diffusion method in the presence of Mg2+. Their hardness is 3.58-16 times greater than that ever reported before for ACC microspheres. The stability of ACC is strongly affected by environmental conditions. The liquid phase and high temperature are not conducive to its stability, but ACC microspheres do have high stability under ambient conditions. After 100 days under such conditions, only a small amount of crystallization occurs, and their spherical shape survives intact. This article provides guidance for the preparation of ACC biomimetic composites, sheds light on the biological function of ACC in crustacean exoskeletons, and improves the theoretical understanding of the mechanism of biomineralization.