How many letters does the top answer for “hard volcanic rock composed of compacted volcanic ash” have?

The most common answer is tufa (4 letters).

Crossword clue: hard volcanic rock composed of compacted volcanic ash

2 answers found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

tufa 4 letters wn-def

hard volcanic rock composed of compacted volcanic ash

tuff 4 letters wn-def

hard volcanic rock composed of compacted volcanic ash