Skarn is a metamorphic rock that has been chemically and mineralogically altered by metasomatism. Metasomatism is the alteration of rocks by hot, chemically-active fluids that flow or diffuse through the rocks and cause recrystallization and compositional change.

Skarn commonly forms around the edges of a magma body that intrudes a nearby rock mass. Rocks formed or altered by the interaction of magma, country rock, reactive fluids, and heat are known as skarn. Other environments of metasomatic activity are also known to produce skarn.

Skarns are not defined by their chemical composition or the types of minerals in the rock. Instead, they are defined by the processes through which the rock was altered.