A Revealer is a magical device used to render invisible writing visible. It has the appearance of an ordinary Muggle eraser, although it has a bright red colour. Hermione Granger bought a Revealer in Diagon Alley sometime in or prior to the summer of 1992. On 1 February, 1993, upon her first inspection of Tom Riddle's old school diary, she used her Revealer, along with an Aparecium spell to detect any hidden writings.