Calcium carbonate latter is especially evident in day-to-day life of people who have hard water. Water in aquifers underground can be exposed to levels of CO much higher than atmospheric. As such water percolates through calcium carbonate rock, the CaCO dissolves according to the second trend. When that same water then emerges from the tap, in time it comes into equilibrium with CO levels in the air by outgassing its excess CO. The calcium carbonate becomes less soluble as a result and the excess precipitates as lime scale. This same process is responsible for the formation of stalactites and stalagmites in limestone
Amorphous calcium carbonate following Ostwald's step rule, which states that the least stable polymorph will precipitate first. But while ACC is the first product to precipitate, it rapidly transforms into one of the more stable polymorphs within seconds. When in pure CaCO, ACC transforms within seconds into one of the crystalline calcium carbonate polymorphs. This transformation from amorphous to crystalline is proposed to be a dissolution-reprecipitation mechanism. Despite ACC’s highly unstable nature, some organisms are able to produce stable ACC. For example, the American Lobster "Homarus americanus", maintains stable ACC throughout its yearly molt cycle. Studies of biogenic ACC have also shown that