Our work focuses on the earliest events of homo-/heterogeneous nucleation from an initial supersaturated solution to the subsequent growth of nuclei. The combined use of conductance, together with hydrogen and calcium ISEs has provided new insights into the mechanisms of crystal nucleation and phase stability. We propose that two types of ACP are formed during HAP nucleation. The initial subcritical calcium-phosphate ion clusters form an amorphous [CaHPO4·xH2O] phase (ACP-1), which transforms to amorphous [Ca3(PO4)2] (ACP-2), and subsequently to HAP. This study is a major step forward in our understanding of the earliest nucleation events in vitro and in vivo. Additives may influence HAP nucleation by interacting with ACP clusters during the early induction period.