The skeletons of many organisms are composed of high-magnesian calcite (10-40 mol.-% Mg), which is not stable under equilibrium conditions. Here, high magnesian calcite particles (>20 mol.-%) are produced at ambient temperature in the presence and absence of additives. Most importantly these particles are shown to initiate from an amorphous precursor phase. The Figure shows the typical dumb-bell morphology of magnesian calcite spherulites.