In the novel, the diadem is inadvertently destroyed by Vincent Crabbe's Fiendfyre inside the Room of Requirement. In Harry Potter and the Deathly Hallows: Part 2, Harry stabs the diadem with a basilisk fang and Ron kicks it to the Fiendfyre (then created by Gregory Goyle).

When did Harry destroy the diadem?

Rowena Ravenclaw's diadem: by Crabbe accidentally by a fiendfyre in Harry Potter and the Deathly Hallows part 2. Harry Potter (specifically the soul fragment within him): by Voldemort using a Avada Kedavra in Harry Potter and the Deathly Hallows part 2.

How does the diadem get destroyed?

Bizarrely, the diadem was accidentally destroyed by the son of a Death Eater. Vincent Crabbe unleashed the powerful Fiendfyre Curse, which quickly blazed out of control – destroying both the diadem and Crabbe himself. Gregory Goyle and Draco escaped thanks to Harry, Ron and Hermione.