What is the X in X-mas?
"metallic iron disulfide, fool's gold," 1550s, from Old French pyrite (12c.), from Latin pyrites, from Greek pyrites lithos "stone of fire, flint" (so called because it glitters), from pyrites "of or in fire," from pyr (genitive pyros) "fire" (see fire (n.)). Related: Pyritic.
A silver to yellow, metallic, cubic mineral. Pyrite often crystallizes in cubes or octahedrons but also occurs as shapeless masses of grains. It occurs in most types of rocks, and is used as a source of iron and in making sulfur dioxide. It is a polymorph of marcasite. Because of its shiny look and often yellow color, it is sometimes mistaken for gold and for this reason is also called fool's gold. Chemical formula: FeS2.