Skarns or tactites are coarse-grained metamorphic rocks that form by replacement of carbonate-bearing rocks during regional or contact metamorphism and metasomatism. Skarns may form by metamorphic recrystallization of impure carbonate protoliths, bimetasomatic reaction of different lithologies, and infiltration metasomatism by magmatic-hydrothermal fluids. Skarns tend to be rich in calcium-magnesium-iron-manganese-aluminium silicate minerals, which are also referred to as calc-silicate minerals. These minerals form as a result of alteration which occurs when hydrothermal fluids interact with a protolith of either igneous or sedimentary origin. In many cases, skarns are associated with the intrusion of a granitic pluton found in and around faults or shear zones that commonly intrude into a carbonate layer composed of either dolomite or limestone. Skarns can form by regional or contact metamorphism and therefore form in relatively high temperature environments. The hydrothermal fluids associated with the metasomatic processes can originate from a variety of sources; magmatic, metamorphic, meteoric, marine, or even a mix of these. The resulting skarn may consist of a variety of different minerals which are highly dependent on both the original composition of the hydrothermal fluid and the original composition of the protolith.

If a skarn has a respectable amount of ore mineralization that can be mined for a profit, it can be classified as a skarn deposit.

Skarn is an old Swedish mining term originally used to describe a type of silicate gangue, or waste rock, associated with iron-ore bearing sulfide deposits apparently replacing Palaeoproterozoic age limestones in Sweden's Persberg mining district.

Skarns are composed of calcium-iron-magnesium-manganese-aluminum silicate minerals. Skarn deposits are economically valuable as sources of metals such as tin, tungsten, manganese, copper, gold, zinc, lead, nickel, molybdenum and iron.

A skarn is formed by a variety of metasomatic processes during metamorphism between two adjacent lithologic units. Skarns can form in almost any rock type such as shale, granite, or basalt but the majority of skarns are found in carbonate rocks containing limestone or dolomite. It is common to find skarns near plutons, along faults and major shear zones, in shallow geothermal systems, and on the bottom of the sea floor. The specific mineralogy of skarns are highly related to the mineralogy of the protolith.

Skarn mineralogy is dominated by garnet and pyroxene with a wide variety of calc-silicate and associated minerals, including idocrase, wollastonite, actinolite, magnetite or hematite, epidote and scapolite. Because skarns are formed from silica-rich aqueous fluids replete with incompatible elements, a variety of uncommon mineral types are found in skarns, such as: tourmaline, topaz, beryl, corundum, fluorite, apatite, barite, strontianite, tantalite, anglesite, and others.

Skarns can be subdivided depending on specific criteria. One way to classify a skarn is by its protolith. If the protolith is of sedimentary origin, it can be referred to as an exoskarn and if the protolith is igneous, it can be called an endoskarn.

Further classification can be made based on the protolith by observing the skarn's dominant composition and the resulting alteration assemblage. If the skarn contains minerals such as olivine, serpentine, phlogopite, magnesium clinopyroxene, orthopyroxene, spinel, pargasite, and minerals from the humite group, it is characteristic of a dolomitic protolith and can be classed as a magnesian skarn. The other class, called calcic skarns, are the replacement products of a limestone protolith with dominant mineral assemblages containing garnet, clinopyroxene, and wollastonite.