First Hokage's Necklace

The necklace worn by Naruto Uzumaki is a special crystalline necklace once worn by Hashirama Senju, the First Hokage. Were one to sell it, three mountains could be bought with the profits.

History
Hashirama gave it to his granddaughter, Tsunade, several decades before the start of the series. She in turn gave it to both her younger brother, Nawaki, and her lover, Dan, in the hopes that it would help them achieve their dreams of becoming Hokage. Both died soon after receiving the necklace during the Second Great Shinobi World War, returning the necklace to Tsunade's possession and leading others to believe the necklace was cursed. Believing this, Tsunade kept the necklace for herself for the next few years and lost faith in those who dream of becoming Hokage.

In Part I of the series, Tsunade meets Naruto Uzumaki. Not only does he dream of becoming Hokage but he also bears a resemblance to Nawaki and Dan. Deciding to tempt fate one more time, Tsunade makes a bet with Naruto: if he can master the Rasengan in a week, therefore proving himself worthy of the position of Hokage, she will give him her necklace. He agrees and is able to accomplish the task, prompting Tsunade to put faith in the title of Hokage and the necklace one more time.

In Part II, when Naruto loses control upon advancing to a six-tailed transformation, the necklace activated in an attempt to stop him. However, due to the overpowering influence of the Nine-Tails, Naruto destroyed the necklace. Yamato claims that without the necklace anymore, it would be impossible for him to help Naruto suppress the fox any longer, were it to resurface.

Abilities
A power unique to the First Hokage was the ability to control jinchūriki and the tailed beasts. This ability was enhanced by the necklace, which responded to his chakra, allowing him to control the chakra of the Tailed Beasts. It is said that the First Hokage's Hokage-style Sixty-year-old Technique - Kakuan Entering Society with Bliss-bringing Hands responds to this necklace to suppress the Nine-Tailed Demon Fox's chakra when it influences Naruto. Yamato at some point in time added a seal to the necklace that would activate when Naruto called upon too much of the Nine-Tails' chakra, attempting to restrain him and alerting Yamato to what was happening.

Trivia

 * In the anime, in likely a dubbing mistake, Jiraiya states that there were only two like it in the world.