1660s (trans.), "kindle or set on fire, cause to burn," from Latin ignitus, past participle of ignire "set on fire, make red hot," from ignis "fire" (see igneous). Attested earlier as an adjective (1550s). Intransitive sense of "catch fire, begin to burn" is from 1818. Related: Ignited; igniting.
"metallic iron disulfide," occurring naturally in cubes and crystals, "fool's gold," 1550s, from Old French pyrite (12c.), from Latin pyrites, from Greek pyritēs lithos "stone of fire, flint" (so called because it glitters), from pyritēs "of or in fire," from pyr (genitive pyros) "fire" (from PIE root *paewr- "fire"). Related: Pyritic.
It is the hypothetical source of/evidence for its existence is provided by: Old Persian atar "fire;" Latin ater "black" ("blackened by fire"), atrox "frightful" ("of fiery or threatening appearance").