"A tiara said to grant wisdom to the wearer. It was lost for centuries before it became a Horcrux."— Pottermore

The Lost Diadem was the only known relic to belong to Rowena Ravenclaw, the founder of Ravenclaw House at Hogwarts School of Witchcraft and Wizardry. 

It was said to enhance the wisdom of its wearer, which is Ravenclaw House's most treasured attribute. It had a blue sapphire shaped like an oval. The diadem was later stolen by her daughter, Helena, a fact Rowena kept secret until her death bed.

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