This study investigated how the rate of addition of the phosphorus-fluoride precursor affects the sustainable synthesis of fluorapatite (FAp) from waste eggshells. A co-precipitation method and different precursor addition rates of 0.5 and 5 mL/min were used to determine their influence on the phase composition, crystallinity, and morphology of the synthesized materials. XRD and FTIR analyses showed that slow addition (0.5 mL/min) promoted the formation of highly crystalline, predominantly phase-pure fluorapatite (93%), while fast addition (5 mL/min) led to supersaturation-induced nucleation, facilitating the stabilization of whitlockite (61%). SEM analysis showed that the fluorapatite crystals were well-shaped and homogeneous, with little agglomeration at 0.5 mL/min. However, when the addition was accelerated, the particles formed irregular structures with greater agglomeration and structural heterogeneity.