Kieserite is a highly unstable magnesium sulfate mineral (MgSO₄·H₂O). It has a vitreous luster and it is colorless, grayish-white or yellowish. Its hardness is 3.5 and it has a monoclinic crystal system. It is used in the production of Epsom salt and is named after Dietrich Georg von Kieser (Jena, Germany 1862).