tuff is a fine-grained volcanic rock created when fine, ashy material ejected during a volcanic eruption settles, accumulates, and through time becomes progressively hardened. Tuffs vary in composition and colour, depending on the chemistry of the parent volcano but all comprise several silicate minerals (see geology) and so have plenty of potential nutrients for the vine. Geologically, young tuffs can still be relatively soft and easily weathered. Tuffaceous soils, such as those found in parts of southern Italy and sicily, can therefore be deep and fertile. The relative softness of young tuff is exploited in places such as tokaj, eger, and Bükk in Hungary for excavating extensive wine cellars. Confusingly, some wine literature refers to tuff as tufa, which is geologically quite different. In Italy, tufo is sometimes used rather inexactly to refer to certain limestone soils as well as to those of volcanic origin.
