Skarn rocks are a component of tephra ejected during the 1944 eruption of Vesuvius. Three different types were recognized: 1. melilite-, 2. phlogopite- and 3. periclase-bearing. The presence of well preserved melt and fluid inclusions and fresh interstitial glass, allowed the reconstruction of the processes occurring during the formation of skarn rocks and the establishment of the P-T-X conditions within the magma chamber wall rocks. Skarn formed at temperatures of about 1000 and 800°C in Type-1 and Type-2 respectively, and at pressures of about 100 MPa. These rocks record in-situ endoskarn genesis at the interface between magma and carbonate rocks in which bimetasomatic diffusion of constituents down chemical potential gradients between magma and carbonate rocks (reciprocal diffusion metasomatism) takes place. Skarn rocks constitute a transition zone between the carbonate country rocks and magma chamber, where the well-defined reaction zones are due to melts infiltrating porous decarbonating rocks. Magmatic melts are modified by addition of Ca and Mg. These melts metasomatize the carbonates inducing skarn reactions. The modified melts during their differentiation may exsolve hypersaline fluids. Different metasomatising agents (melt, melt + hypersaline fluids and possibly hypersaline fluids) produce the different types of skarn facies that are transitional to thermometamorphic marbles.