How many letters does the top answer for “a soft, earthy, dark-colored rock or clay derived from the alteration of basalt.” have?

The most common answer is wacke (5 letters).

Crossword clue: a soft, earthy, dark-colored rock or clay derived from the alteration of basalt.

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

wacke 5 letters web-def

A soft, earthy, dark-colored rock or clay derived from the alteration of basalt.

wacky 5 letters web-def

ludicrous, foolish