Magnesite (MgCO3) is a carbonate mineral of magnesium. It is similar to limestone, but much rarer. Limestone is pure CaCO3, dolomite is a mixture of CaCO3 and MgCO3, magnesite is pure MgCO3. Those three kinds of carbonates are very difficult to distinguish. While limestone and dolomite typically are sedimentary rocks formed mostly by biogene processes, is magnesite mostly an evaporite mineral in sedimentary rocks. It also occurs as veins in and an alteration product of ultramafic rocks, serpentinite and other magnesium rich rock types. This kind of magnesites in contact and regional metamorphic terranes are often cryptocrystalline and contain silica as opal or chert.

Magnesite is mined for the use as magnesium oxide lining in furnaces. For this it is burned in the presence of charcoal (reduced) to produce MgO or periclase. In such furnaces magnesite is often used in conjunction with lime as a slag former in steelmaking. The use of magnesite protects the magnesium oxide lining. It is also used as catalyst and filler in the production of synthetic rubber and in the preparation of magnesium chemicals and fertilizers.

Magnesite is typically mined or quarried in opencast mines. The low price in general does not allow expensive underground mining operations. Nevertheless there are some underground magnesite mines, especially historic mines.