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Crossword clue: a fine-grained metamorphic rock formed by the action of heat on clay rocks

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

Possible answers

hornfels 8 letters wn-def

a fine-grained metamorphic rock formed by the action of heat on clay rocks

hornstone 9 letters wn-def

a fine-grained metamorphic rock formed by the action of heat on clay rocks

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Related clues

Another word for hornstoneA siliceous stone, a variety of quartz, closely resembling flint, but more brittle; -- called also chert.