Definition of "marcasite"
noun
countable and uncountable, plural marcasites
(historical) Any of various metal sulfides, usually iron sulfide minerals.
Quotations
If he wants to speak as a physician, however, he must say, this marcasite is the man's disease, hence it will cure him.
1999, Paracelsus, “Opus Paramirum”, in Nicholas Goodrick-Clarke, transl., Essential Readings, North Atlantic Books, page 95