Marcasite is the lesser known companion of the famous mineral Pyrite. Marcasite has the same chemical formula as Pyrite, but crystallizes in a different crystal system, thereby making it a scientifically distinct mineral species. Aggregates of iron sulfide (FeS₂) where the crystal structure cannot be determined without complex analysis may be wrongly labeled by dealers and collectors. Some Marcasite specimens are labeled as Pyrite, and some Pyrite specimens as Marcasite. Pyrite used as a gemstone is also improperly termed in the jewelry trade as Marcasite. This is wholly incorrect, as Marcasite is not used as a gemstone.

Marcasite is the most prevalent mineral to experience a condition known as pyrite decay, in which a specimen will slowly disintegrate into a white powder. Little is known about this detrimental condition. It only effects certain specimens at random, while other specimens remain unaffected. However, certain localities are more prone to this condition than others. When a specimen goes through pyrite decay, the sulfur atoms free themselves and form an acid powder that attacks other sulfide minerals and mineral labels. It is most important to remove an afflicted specimen from other minerals, to prevent this condition from spreading.