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1.Magnesium ion is essential to the basic nucleic acid chemistry of life, and thus is essential to all cells of all known living organisms. Plants have an additional use for magnesium in that chlorophylls are magnesium-centered porphyrins. Many enzymes require the presence of magnesium ions for their catalytic action, especially enzymes utilizing ATP, or those which use other nucleotides to synthesize DNA and RNA.
2.As the metal
Magnesium is the third most commonly used structural metal, following steel and aluminium.
Magnesium compounds, primarily magnesium oxide, are used mainly as refractory material in furnace linings for producing iron, steel, nonferrous metals, glass and cement. Magnesium oxide and other compounds also are used in agricultural, chemical and construction industries. As a metal, this element's principal use is as an alloying additive to aluminium with these aluminium-magnesium alloys being used mainly for beverage cans.
3.Food sources
Green vegetables such as spinach provide magnesium because the center of the chlorophyll molecule contains magnesium. Nuts (especially almonds), seeds, and some whole grains are also good sources of magnesium.
Although magnesium is present in many foods, it usually occurs in dilute form. As with most nutrients, daily needs for magnesium are unlikely to be met from a single serving of any single food. Eating a wide variety of foods, including five servings of fruits and vegetables daily and plenty of whole grains, helps to ensure an adequate intake of magnesium.
Because magnesium readily dissolves in the water used to refine foods, and also in the water-rich parts of certain foods which are removed during refining, the magnesium content of many refined foods is low. Whole-wheat bread, for example, has twice as much magnesium as white bread because the magnesium-rich germ and bran are removed when white flour is processed. The table of food sources of magnesium suggests many dietary sources of magnesium.
Water can provide magnesium, but the amount varies according to the water supply. "Hard" water contains more magnesium than "soft" water. Dietary surveys do not estimate magnesium intake from water, which may lead to underestimating total magnesium intake and its variability.
Too much magnesium may make it difficult for the body to absorb calcium. Not enough magnesium can lead to hypomagnesemia as described above, with irregular heartbeats, high blood pressure (a sign in humans but not some experimental animals such as rodents), insomnia and muscle spasms. However, as noted, symptoms of low magnesium from pure dietary deficiency are thought to be rarely encountered.
Following are some foods and the amount of magnesium in them

spinach (1/2 cup) = 80 milligrams (mg)

peanut butter (2 tablespoons) = 50 mg

black-eyed peas (1/2 cup) = 45 mg

milk: low fat (1 cup) = 40 mg
The U.S. RDA/RDV is 400 mg of magnesium.

Magnesium compounds, primarily magnesium oxide, are used mainly as refractory material in furnace linings for producing iron, steel, nonferrous metals, glass and cement. Magnesium oxide and other compounds also are used in agricultural, chemical and construction industries. As a metal, this element's principal use is as an alloying additive to aluminium with these aluminium-magnesium alloys being used mainly for beverage cans.

3 examples of magnesium can be use to make:
-Combined in alloys, this metal is essential for airplane and missile construction.
-Removal of sulfur from iron and steel.
-Photoengraved plates in the printing industry.