Amorphous and liquid precursors of calcium carbonate are believed to be central species of biomineralization, which serves as an important inspiration for materials chemists in the quest for new and improved organic-inorganic hybrid materials. It has become increasingly clear that magnesium ions exhibit an important function through kinetic stabilization of the metastable precursors. We show that they additionally tune the wettability of liquid precursors of CaCO3, which is a crucial requirement for successful mineralization of proteinaceous organic matrices. Moreover, tunable wettability offers straightforward means to control mineralization sites in organic-inorganic hybrids.