<p>Mussel shells, commonly discarded by restaurants, households, and aquaculture facilities, cause environmental problems due to their slow decomposition and improper disposal, such as dumping into the sea or landfills. As part of sustainability-oriented strategies, reusing such biogenic waste as a raw material offers both ecological and economic benefits. This study investigates the potential of mussel shell powder as a sustainable calcium oxide (CaO) source to replace calcite in opaque, matte, and transparent ceramic tableware glazes X-ray fluorescence (XRF) analysis revealed that, apart from their organic content, the chemical composition of mussel shells is highly similar to that of calcite. While calcite naturally contains the calcite calcium carbonate (CaCO₃) phase, mussel shells are composed of calcium carbonate and its polymorph, aragonite. During firing, the aragonite phase transforms into calcite and subsequently decomposes into CaO. Thermal and phase analyses indicated that the incorporation of mussel shell promotes anorthite formation, resulting in improved microhardness and enhanced thermal stability of the glazes. Additionally, the use of mussel shell did not adversely affect surface gloss or color; all compositions exhibited ∆E values below 1, indicating a negligible color change. Overall, mussel shells can be effectively utilized as an eco-friendly and cost-efficient CaO source in glaze formulations. This approach contributes to waste valorization and supports the development of sustainable ceramic materials.</p> Graphical Abstract <p></p>