Crossword clue: a composite stone (quartz, schorl, and hornlende) in the walls of tin and copper lodes.
1 answer found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).
Possible answers
capel
5 letters
web-def
A horse; a nag. [Obs.] Chaucer. Holland.