Green mussel and crab shells are natural sources of CaCO₃, which is widely used as a bioceramic for biomedical applications, although they are commonly disposed of in landfills. The improper disposal of green mussel and crab shells can cause environmental pollution, reducing the quality of life in the community. Many studies have reported the preparation of CaCO₃ from green mussels and crab shells. However, there are limited studies comparing the characteristics, including the crystal phase obtained, weight percentage (%) of crystal, crystal size, crystal system, and elemental composition of CaCO₃ from green mussel shells, crab shells, and commercial CaCO3. The objective of this research was to compare the calcium carbonate properties formed from green mussel (PMS) and crab (PCS) shells to commercial CaCO₃. Green mussel and crab shells were crushed to powder and were calcined at 900 °C for 5 h. Precipitated Calcium Carbonate (PCC) was synthesized from calcined green mussel and crab shells using a solution of 2M HNO₃, NH₄OH, and CO₂ gas. The effect of setting parameters on the synthesized product was analyzed using XRD and SEM-EDX methods. This study shows that the chemical composition of PMS is nearly identical to that of commercial CaCO₃, where no contaminants were identified. In contrast, PCS has N components other than Ca, C, and O. Furthermore, the predominance of the vaterite crystal phases in PMS and PCS, with respective weight percentages of 91.2% and 98.9%, provides a benefit for biomaterial applications. The crystallite sizes of vaterite in PMS, PCS, and calcite in commercial CaCO₃ are 34 nm, 21 nm, and 15 nm, respectively.