"I only meant - do you know where it is? Have you ever seen it?"
"Seen it? Nobody has seen it in living memory! Long since lost, boy!" (The Deathly Hallows, 601)In the book, it is Harry who puzzles it out for himself and realizes he must talk to a ghost to find the lost diadem. Harry goes in search of the ghost of Ravenclaw house, the Grey Lady, who is Helena Ravenclaw, daughter of Rowena Ravenclaw, founder of Ravenclaw house.
"Sorry, but what is a diadem?" asked Ron.
"It's a kind of crown," said Terry Boot. "Ravenclaw's was supposed to have magical properties, enhance the wisdom of the wearer." (The Deathly Hallows, 584)After stealing her mother's diadem, Helena ran away and hid in a forest. When her mother grew ill, she sent the Baron, who loved Helena, to find her so her mother could say her farewells. However, Helena refused to return with him and the Baron, being a "hot-tempered man" stabbed her and then killed himself when he realized what he'd done (The Deathly Hallows, 616). Her untimely death is what caused the stolen diadem to become the Lost Diadem.
The diadem remained hidden in the forest, until the ghost of Helena Ravenclaw told Tom Riddle (Voldemort) about it: "I had...no idea...He was...flattering. He seemed to...to understand...to sympathize..." (The Deathly Hallows, 617).
Harry finds the diadem in the Room of Requirement...which can change to suit the needs of the person seeking it - in this case a place to hide things.
It was still hot, blackened with soot, but as he looked at it closely he was just able to make out the tiny words etched upon it: Wit beyond measure is man's greatest treasure. A bloodlike substance, dark and tarry, seemed to be leaking from the diadem. Suddenly Harry felt the thing vibrate violently, then beak apart in his hands, and as it did so, he thought he heard the faintest, most distant scream of pain, echoing not from the grounds or the castle, but from the thing that had just fragmented in his fingers. (The Deathly Hallows, 635)I personally am a huge fan of all of the myths J.K. Rowling sets up in Harry Potter that end up being true, such as this story of the lost diadem, the Tale of the Three Brothers (Deathly Hollows), and the Chamber of Secrets.
Up next in this Harry Potter themed A to Z challenge is one of my favorite magical objects, the Marauder's Map!