Rock produced by the settling of volcanic ash, usually originating from hot clouds that have been ejected from an erupting volcano. Ash particles can become fused by the intense heat. Some forms can be used to make stone tools. Aka welded tuff.
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igneous rock (... tools, and include lavas, ignimbrites and obsidian.)
vitrification (Process of forming stone from some other material; see ignimbrite for an example.)