Tuff is the rock formed when volcanic ash becomes compacted and cemented.
Volcanic ash consists mainly of tiny fragments of volcanic glass, crystals, and older rock blasted into the atmosphere during an eruption. When this material settles, thick deposits can accumulate on land or beneath water. Burial, compaction, and chemical cementing can then turn the loose ash into tuff.
Tuff can vary greatly in appearance and composition. Some tuffs are pale and lightweight, while others contain dark crystals or larger fragments called lapilli. If the eruption is especially hot, ash particles may weld together while still hot, producing welded tuff.
Tuff is not the same as ordinary ashstone in every usage, and it should not be confused with pumice. Pumice is a highly vesicular volcanic rock formed from frothy lava, whereas tuff is defined by its origin from consolidated volcanic ash.