First Hokage's Necklace

The was a necklace originally belonging to Hashirama Senju, the First Hokage. It was made from a special and augmented his abilities to capture and control tailed beasts. If it was to be sold, three mountains containing gold mines could be bought with the profits.

History
The First Hokage's granddaughter, Tsunade, inherited it several decades before the start of the series. She in turn gave it to both her younger brother, Nawaki, and her lover, Dan Katō, in the hopes that it would help them achieve their dreams of becoming Hokage. Both died soon after receiving the necklace, 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 years and lost faith in those who shared Nawaki and Dan's dream of becoming Hokage. Over the years, despite Tsunade's gambling addiction, she did not wager the necklace, as Jiraiya thought she would have done so long ago.

In Part I, Tsunade met Naruto Uzumaki. Not only did he dream of becoming Hokage, but he also bore some resemblance to both Nawaki and Dan. Deciding to tempt fate one more time, Tsunade made a bet with Naruto: if he could master the Rasengan in a week, therefore proving himself worthy of the position of Hokage, she would give him her necklace; if he failed, she wins his money-filled Gama-chan. He agreed and was 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 lost control upon advancing to his six-tailed form, the necklace activated in an attempt to stop the transformation. However, due to the overpowering influence of the Nine-Tailed Demon Fox, it broke the necklace. Yamato, who had inherited Hashirama's Wood Release and thus his ability to control the Nine-Tails, claimed that without the necklace, it would be impossible for him to help Naruto suppress the Nine-Tails' chakra any longer, were it to resurface. However, this may not be completely true, as Yamato once again tried to suppress the transformation while they were in the Falls of Truth, without the necklace's assistance.

Abilities
Hashirama was able to control a tailed beast and, by proxy, its jinchūriki. 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 Hashirama's Hokage-Style Sixty-Year-Old Technique — Kakuan Entering Society with Bliss-Bringing Hands responds to this necklace to suppress the Nine-Tails' 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.

Utilised Techniques

 * Hokage-Style Sixty-Year-Old Technique — Kakuan Entering Society with Bliss-Bringing Hands
 * Tailed Beast Sealing

Trivia

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