Researchers developed a unified model of the formation mechanism of spherical carbonate concretions, which often contain exceptionally well-preserved fossils. The carbon in the carbonate originates from the organisms preserved inside the concretions, and the surrounding muddy matrix limits diffusion and permeability, and thus causes supersaturation of carbonate at a reaction front. Calcite precipitation occurs several orders of magnitude more rapidly than previously recognized in concretions. This understanding may have practical applications in sealing technology.