The ocean’s chemical system is complicated and always changing. The minerals are dissolved in water and are in solution until either chemical or biological reactions in the oceans precipitate the different compounds.

Organisms that live in the marine environment tend to use two types of compounds, calcium carbonate (CaCO₃) and silica dioxide (SiO₂). Biological processes can actually transform the ions into microscopic minerals that grow with the organism. Calcium carbonate is a mineral called calcite. When incorporated into a biological system it is usually in a "disordered" form called aragonite. Most mollusks use calcium carbonate in their shells.

Silica dioxide is also a common mineral, in the form of quartz. When used in the biological system it is also "disordered" and is called opaline silica. Siliceous sponges use opaline silica for its skeleton.

Calcium carbonate shells

Venus flower basket, a sponge