Take another look at “rubiginous” — you might see part of a familiar word hidden inside it. “Rubiginous” shares Latin roots with another reddish word: “ruby.” Something that’s rubiginous has the reddish-brown color of rust or may be covered in actual rust. Rust is the result of a reaction between iron and oxygen combined with the presence of water or moisture. Given enough time, all iron will convert to rust and eventually disintegrate.