The Girls of Great Britain and Ireland Tiara

 This tiara is not identical to the real one. This is a design of a replica set in gold while the real one is set in platinum.

As you guessed by now, it was gifted by a commitee of women from the UK to Queen Mary for her wedding. It consists of a scroll and festoon design, sitting on a base of round and lozenge diamonds. The original design was toped with pearls. The single diamonds came from a dismantled tiara Queen Mary previously owned. Queen Mary was known for her jewel creativity, often giving them a makeover. The base is detachable, and that's how Queen Elizabeth received it as wedding present. It became shorter and lighter, fit for a Princess. OH! And It can also be worn as a necklace. You can see it still today, on British currency, stamps and frequently on Her Majesty's head, as it is her favorite. Following Queen Mary's death, QEII inherited all her jewels, thus having full acess to the vault. She then discovered it came with a base, as she said while being filmed for a portrait. She has never worn the top pearls on it; she prefers the jewels as they are. Said pearls were placed on the arches of the Cambridge Lover's Knot tiara, the famous tiara Princess Diana loved, that now has found a new wearer, the Duchess of Cambridge.The Great Britain &Ireland tiara has become a symbol of the UK and it is now an iconic piece in the whole world.

Side fact: The tag says crown, but this is clearly a tiara. There seems to be a big minsunderstanding between the term crown and tiara. It's like we're calling a sandal a boot. Both are shoes, but aren't the same thing, otherwise they wouldn't have different names.