To mark its 140th anniversary, Bulgari has created its most expensive piece of jewelry: a 200-carat diamond necklace priced at 40 million euros.

Jean-Christophe Babin, the house’s chief executive, said the necklace, the Serpenti Aeterna, represented the achievement of a significant goal: When he took the reins in 2013, the house’s tradition of high jewelry had lost its luster so he decided to “push forward the best of our expertise and our most beautiful gems, to create the jewelry of the century.”

The $42.9 million necklace — worn by Priyanka Chopra Jonas, one of the house’s celebrity ambassadors — was the star of the 500-piece Aeterna collection of jewelry, watches, perfume and accessories presented in May during a private party at the Baths of Diocletian in Rome. (Bulgari, which was founded and still has its headquarters in the Italian capital, said the Latin word aeterna, or eternal, was chosen as the collection’s title to echo Rome’s nickname, Eternal City.)

In 2022, the house began considering how it should celebrate its milestone. “I was thinking of producing a piece featuring colored gemstones,” something the house has been known for since the 1960s, said Lucia Silvestri, its jewelry creative and gems buying director. But she saw a rough diamond of more than 200 carats during a trip to Hong Kong that year and, she said during a recent interview, “I started to dream.”

Ms. Silvestri felt that the diamond, mined in Lesotho, couldn’t be set intact. “It would be too big and look too heavy,” she said. “It wouldn’t look graceful.”

The gem was cut into seven stones, which she described as “perfectly pear-shaped diamonds, each D-Flawless, not too big, very elegant.” In the gem industry, a diamond must be of exceptional color and clarity to be ranked D-Flawless.

The seven drops — which total 140 carats to match the number of anniversary years — were attached to a platinum choker set with 698 diamond baguettes. A serpent’s head, the house’s signature motif, formed the necklace’s clasp.

Bulgari said it took 2,800 hours, from the initial design work through production, to create the piece. The necklace has been sold, but the brand would not identify the buyer.

Ms. Silvestri said that she sometimes had a client in mind when she created high jewelry designs, but not when it came to the Serpenti Aeterna. “Honestly, we didn’t think about it,” she noted. “I just wanted to make the most beautiful piece of my life.”