Merriam-Webster Collegiate Dictionary - pyrite

 noun  Etymology: Latin ~s  Date: 1756 a common mineral that consists of iron disulfide, has a pale brass-yellow color and metallic luster, and is burned in making sulfur dioxide and sulfuric acid PYRITES  noun  (plural ~)  Etymology: Latin, flint, from Greek pyrites of or in fire, from pyr fire  Date: 1543 any of various metallic-looking sulfides of which pyrite is the commonest  • pyritic adjective