Magnesium is a mineral, one of six alkaline earth metals - along with beryllium, calcium, strontium, barium and radium - that make up the second column of the periodic table.  In nature, it can only be found as a salt, that is, in combination with other minerals such as magnesium carbonate, magnesium sulphate, magnesium citrate, to name but a few.  Elemental or free magnesium that is not combined with another element is a highly flammable, lightweight, shiny gray solid that can only be produced artificially.