Vincent Crabbe destroyed the diadem when he used fiendfyre (cursed fire) on it, while trying to kill Harry Potter (along with Ron Weasley and Hermione Granger).
Crabbe (one of Draco's friends) starts fiendfyre (cursed fire) in the Room of Requirement. He is trying to kill Harry, Ron, and Hermione. However, the fire gets out of control and consumes the room,...
Its like a crown or a tiara inscribed with the words "Wit, beyond measure, is man's greatest treasure." Rowena Ravenclaw used to own this when she was alive. It it one of the horcruxes. It is...