atomic number 12, magnesium, Mg - a light silver-white ductile bivalent metallic element; in pure form it burns with brilliant white flame; occurs naturally only in combination (as in magnesite and dolomite and carnallite and spinel and olivine)

mineral - solid homogeneous inorganic substances occurring in nature having a definite chemical composition

ruby spinel, spinel ruby - a spinel used as a gemstone (usually dark red)

Ceylonite, pleonaste - a dark-colored spinel containing iron