The existence of a metastable field in the precipitation of calcium carbonate is an experimental fact. Indeed, the saturation in relation to the different anhydrous forms of CaCO3 (calcite, aragonite and vaterite) is often extensively passed without any precipitation occurring. Through experimental exploration of the metastable field by the LCGE method and the use of new thermodynamic data of CaCO3 hydrated forms, a new diagram of thermodynamic evolution of the calco-carbonic system has been validated. Above 20 degreesC, the solubility product of calcium carbonate monohydrate (MCC) constitutes a necessary lower limit for deferred spontaneous germination. In contrast, precipitation with homogeneous predominance will happen instantaneously when the solubility product of amorphous calcium carbonate (ACC) is reached. When the ionic activity product of the solution, (Ca2+)(CO32-), stabilizes between solubility products of monohydrated and amorphous forms, the germination time and the precipitation kinetics highly depend on the wall nature. Germination can only be predominantly heterogeneous. These two hydrated forms (ACC and MCC) constitute precursors to CaCO3 precipitation. They are very unstable and are quickly transformed to give the anhydrous forms of calcium carbonate.