Skarn is a crystalline-textured, contact metamorphic rock. It forms by significant heating and addition of elements (metasomatism) to country rocks in the immediate vicinity of an igneous intrusion (batholith, stock, sill, dike, laccolith). Carbonate rock skarns are frequently called tactites.
Shown below are samples of an andradite-diopside skarn from Nevada having light reddish-brown andradite garnets (ideally Ca3Fe2Si3O12), dark greenish diopside (a Ca-Mg pyroxene), silver-colored molybdenite (molybdenum sulfide - MoS2), a little quartz (silicon dioxide - SiO2), and a little scheelite (calcium tungstate - CaWO4). The molybdenite content is high enough to qualify these rocks as molybdenum ores.
Geology - contact metamorphosed limestones of the Comus Formation (Upper Cambrian-Lower Ordovician) against the Osgood Mountain Stock, an early Late Cretaceous (92 Ma) granodiorite intrusion. The garnet-rich skarn zone around the Osgood Mountain Stock is moderately rich in tungsten (W) and occasional molybdenum (Mo).
Locality - Reilly Mine, located in section 9, T38N, R42E, Potosi District, eastern side of the Osgood Mountains (Osgood Range), eastern Humboldt County, northern Nevada, USA (= approximately 41° 11’ 21” North, 117° 15’ 03” West).
Skarn (field of view ~2.0 cm across) with andradite garnet (reddish brown), diopside pyroxene (green), and molybdenite (silvery gray).
Skarn (above & below; field of view ~4.6 cm across) with andradite garnet (reddish brown), diopside pyroxene (dark green), molybdenite (silver), and scheelite (white). How do I know the white stuff is scheelite? Scheelite fluoresces bright blue under ultraviolet light (black light) - see photo below.
Some info. provided by Keith Wood.