A very hard, brittle, highly corrosion-resistant, whitish-yellow, metallic chemical element. Iridium is used for the fine-wire electrodes in spark plugs that must operate in engines using fuel with an exceptionally high lead content. iso-octane. An organic compound used as the high reference fuel for rating the antidetonation characteristics of aviation gasoline (CH₁)₁CHCH₁C(CH₁)₁.