Item from harry-potter-deathly-hallows by
Silver tiara Horcrux hidden in the Room of Requirement for decades -- destroyed by Fiendfyre during the Battle of Hogwarts when Crabbe cast cursed fire he couldn't control.
The Grey Lady -- Helena Ravenclaw -- revealed the diadem's location to Harry during the battle. He found it in the Room of Hidden Things, perched on a bust among centuries of discarded objects. Draco, Crabbe, and Goyle followed him in. Crabbe cast Fiendfyre -- a cursed fire that takes the shape of monstrous serpents and chimeras and cannot be extinguished by normal means. The Room of Hidden Things became an inferno. Harry rescued Draco and Goyle on broomsticks. Crabbe died in his own fire. The diadem was destroyed by the Fiendfyre -- one of the few substances besides Basilisk venom that can damage a Horcrux beyond magical repair. The destruction was accidental. Crabbe cast the fire to kill Harry, not to destroy a Horcrux. The irony was lost on everyone at the time.
Elegant silver tiara with a sapphire center, inscribed 'Wit beyond measure is man's greatest treasure.' Tarnished after decades among junk in the Room of Hidden Things.
Also known as: Ravenclaw's Diadem, the Diadem, the lost diadem