Pyrite

PYRITE, noun plural pyrites. [Gr., fire.] Fire-stone; a genus of inflammable substances composed of sulphur and iron or other metal; a sulphuret of iron or other metal.

Hence sable coal his massy couch extends, and stars of gold the sparkling pyrite blends.

[I have anglicized this word, according to Darwin and the French mineralogists; making pyrites a regular plural.]