Magnesium is an essential mineral that can be found in many foods and dietary supplements. It is involved in more than 300 enzymatic reactions in the body and has several important biological functions. It also is used in metallurgy, primarily to produce the metal magnesium carbonate (MgCO3). This form is used in manufacture of heat exchangers, water heaters, and other industrial equipment, as a white pigment in paints and plastics, as an electrolyte in batteries, and in pharmaceuticals and cosmetics. It is also used in glass, ceramics, and tanning leather. Magnesium reacts with alkalies to produce the mineral magnesite and with acids to liberate hydrogen gas, which is used in the production of magnesium metal by the electrolytic reduction process.

Magnesium in a pure state is very soft and brittle and is usually found in the +2 oxidation state in compounds. In nature it exists as the metallic mineral magnesite and as magnesium chloride (MgCl), both of which are found in sedimentary rocks and as the igneous minerals epsomite and tonalite. The magnesium ions in these compounds are strongly bonded and have a net positive electric charge.

The ratio of the stable isotopes of magnesium (mg 24) to aluminum-26, known as the magnesium/aluminum isotopic composition, is important in determining the age of meteorites. The method of dating based on this ratio is called the magnesium-aluminum (Mg/Al) isochron. Because of the large range of mass-dependent fractionation, the d26Mg* values that are used in constructing bulk CAI 26Al-26Mg regression lines (isochron) must be corrected for to obtain high-precision results.