Blue Calcite SphereA sphere the color of a pale winter sky, surface matte and soft to the touch. Blue calcite is calcium carbonate (CaCO) colored by trace clay minerals and structural impurities that shift it from the mineral's natural white toward a quiet blue grey. Calcite is the defining mineral on the Mohs hardness scale at 3soft enough to scratch with a copper coin. Its three perfect rhombohedral cleavage planes give each piece a characteristic way of catching