When Nozomi was a child, Tsuna gave her a medallion called Vongola Locket, it has the Vongola mark on it along with the number “XI”. It can be opened, inside there's another mark with the words "XI Insieme Vongola Famiglia" and an orange winged clam surrounded by six colored pearls who resemble the boss and her guardians. The pearls and the clam can be pressed, when the clam is pressed she can call all the Vongola W.S. at once, otherwise she can call the guardian she wants by pressing the specified pearl.
She didn't remember why her father gave it to her because she knows he doesn’t want to involve her in Vongola's and mafia's stuffs. However, in the last chapters of the Clover's Arc, Tsuna explains it to her. He said when she was little she had insisted on having something like Primo's clock. She was so insistent that he decided to please her daughter, giving her something colorful because she was a child. He also says that he created it to make her understand that it isn't important what is outside but what's inside (the pearls, the bond between her and her friends).