Large crystals of magnesium sulfate are rare, while crusts and massive forms are much more common. Epsomite (the mineral form of magnesium sulfate) forms as a precipitation from vapors (efflorescent) on limestone cave walls and on the walls and timbers of deep-shaft mines. In addition to the solubility of the crystals, the bitter, salty taste of epsomite is very characteristic of the compound. The dust of the salt is irritating to the eyes and throat as may be experienced by exposure to dry lake beds.

In the early 1600s, the sleepy village of Epsom, England was transformed into Britain's first spa town because of its healing waters that contain significant levels of magnesium sulfate. Medical uses of magnesium sulfate include soaking in Epsom salt solutions to relieve aches and pains, and taking the mineral as a dietary supplement to reduce the debilitating effects of chronic asthma and convulsions, particularly during emergencies. By providing magnesium to intracellular fluids, the inorganic central nervous system (CNS) depressant reduces the excess release of acetylcholine and muscle excitability. In addition to its actions as a provider of required electrolytes, oral magnesium sulfate has long been known for its laxative properties and partially explains problems some people experience while drinking from mineral springs or well water in karst topography. Pregnant women sometimes use magnesium sulfate to control preterm labor and toxemia. Overdose (too much magnesium) is possible and can lead to cardiac arrest, pulmonary edema, low blood pressure, and low calcium levels in the body with accompanying loss of bone density. As a cathartic, magnesium sulfate can be administered to reduce the absorption of poisons from the gastrointestinal tract, and there are some indications that magnesium sulfate reduces the risks of cerebral palsy.

Industrial uses for magnesium sulfate include fertilizers, fireproofing, detergents, refractory bricks for lining iron and steel furnaces, tanning and textile dyes, chemical production, and ceramics. Magnesium oxide, as mined or extracted from seawater, acts as the starting point for commercial production of magnesium sulfate. Magnesium sulfate finds additional uses in mining and fermentation, in the pulp and paper industries, and as a salting-out agent for acrylonitrile-butadiene-styrene (ABS) resins. Gardeners use Epsom salts for ensuring strong root structures and facilitating the uptake of chlorophyll, with some foliage plants requiring relatively larger quantities.