From coral reefs to scale on bathroom shower doors, calcium carbonate crystals are ubiquitous. Yet the early stages of CaCO3 crystallization are still poorly understood. A research group led by Helmut Cölfen of the Max Planck Institute of Colloids & Interfaces, in Potsdam, Germany, has now observed pH-dependent formation of thermodynamically stable clusters of about 70 Ca2+ and CO32- ions that the scientists believe are key to crystal formation (Science 2008, 322, 1819). The researchers propose that these clusters aggregate into amorphous CaCO3, which then forms into crystalline material, such as calcite. This suggested pathway is in contrast to the classical view of crystallization, which holds that metastable clusters form randomly, then incorporate single ions to nucleate into amorphous and then crystalline forms. Cölfen and coworkers also observed the stable cluster formation pattern for calcium phosphate and calcium oxalate, suggesting that the proposed mechanism may apply to a range of minerals.