Rowena Ravenclaw’s diadem is the only known relic that once belonged to the co-founder of Hogwarts School of Witchcraft and Wizardry. Etched upon the surface was Ravenclaw’s famous quote: “Wit beyond measure is man’s greatest treasure.” It was said to enhance the wisdom of its wearer, which is Ravenclaw House’s most treasured attribute. It has a blue sapphire shaped like an oval. The diadem was later stolen by her daughter, Helena, a fact Rowena kept a secret from the other founders ’till her death bed.
This is obviously a replica. The original was destroyed by Harry Potter as Tom Riddle had turned it into a Horcrux.