1. The process by which organisms produce skeletal structures containing crystalline or amorphous inorganic substances.
2. The production of partly or wholly mineralized internal or external structures by living organisms.

Organisms use a wide variety of minerals to make their skeletons, including silica, apatite, and several polymorphs of carbonate, in particular aragonite and calcite.

It is unclear, however, why different taxa evolved to use one mineral rather than another. Lineages rarely switched their mineralogy after acquiring skeletons, suggesting that, for most taxa, ambient seawater chemistry does not strongly influence skeletal mineralogy.

Mineralization of most animal skeletons is biologically controlled, occurring in an environment isolated from seawater. As a result, seawater chemistry does not have a direct influence on the mineralogy of most animal skeletons the way it does for biomineralizers that induce mineralization directly from seawater.