Biomineralisation is the process by which living organisms prouce minerals, often to harden or stiffen existing tissues. Examples include silicates in algae, carbonates in diatoms and invertebrates, and calcium phosphates and carbonates in vertebrates. These minerals often form structural features such as sea shells and the bone in mammals and birds. Organisms have been producing mineralised skeletons for the past 550 million years. Other examples include copper, iron and gold deposits involving bacteria. As a result biomineralization is employed in metal extraction.