How do you celebrate a 40th birthday? Get dressed head-to-toe in solid 18-karat gold, that’s how.

Or at least that’s what Casio has done for the anniversary of its fan-favorite G-Shock: Created a one-of-a-kind timepiece called the Dream Project #2, Model G-D001.

“This is the only piece in the world,” said Kikuo Ibe, the Casio engineer known as “the father of G-Shock.”

And to talk about the specialty watch, Mr. Ibe and his G-Shock co-creator, Yuichi Masuda, met on a cloudy November day for an interview at Casio headquarters in Tokyo’s Shibuya ward.

Casio staff members remarked on how unusual it was for the two men, who work in different locations, to be together. Mr. Masuda, 69, the company’s president and chief executive, has his office at the Shibuya headquarters. (He was the team leader in charge of the G-Shock project in the early 1980s.) And Mr. Ibe, 71, works at the research and development center in Hamura, west of Tokyo.

I couldn’t see the actual Dream #2, or even the primary prototype, made of 18-karat solid gold. After being displayed in Singapore, Taiwan and Geneva, it was sent to New York, where Phillips had scheduled an auction Dec. 10, with the proceeds going to the Nature Conservancy, an environmental organization. (The sales estimate was $70,000 to $140,000.)

The winning bidder is to receive a final version of the watch in early 2024.

Instead, I saw another iteration of the Dream #2 prototype, just plated in gold. And I saw a regular model of the Dream Project #1 — “the one we made for the 35th anniversary,” Mr. Ibe said, displaying the gold-plated steel version usually in the headquarters showroom and still available online and in stores.

The actual Dream #1, designed with the traditional G-Shock square case and digital display, was made with a solid 18-karat gold case and bracelet — and was released in a 35-piece limited edition, each priced at 7.7 million yen, or what now would be $51,500. It sold out.

“This project aimed to develop a dreamlike G-Shock,” he said. “And for this second one, it was the first time the designers collaborated with A.I. to create a watch.”

Sixteen designers spent two years on the Dream #2 project, first determining a concept for the timepiece, which has a 45.1-millimeter round case and an analog dial. “For 40 years,” Mr. Ibe said, “G-Shock has gathered data on shock-resistance technology. First, they let A.I. learn about the technology, then they merged the A.I. and designers’ ideas.

“And that would make an original piece with no parallel in the world.”

The watch’s significant characteristics, he said, were its use of metal gears and its electric parts. “Metal gears were used to enable the next-generation solar cells, which can generate electricity even with very little light,” he said.

Mr. Ibe pointed at the model’s skeleton-style dial, complete with a date window. “Actually, you can’t really see it, but the solar panel is underneath,” he said. The watch has 55 rubies in the bearings to reduce wear and tear; on average, a mechanical timepiece has 17.

The prototype was assembled at the Yamagata factory in the north of Japan’s main island, Honshu. (The watch’s development and quality-assurance testing took place in Hamura.)

And the result? “It passed all the tests,” Mr. Ibe said.

The final watch will come in packaging almost as blingy as the design: a gold-color metal tower whose four sides have a cutout pattern of G-Shock model numbers. The front panel can be slid up to reveal the watch, and there is a small light inside. “When the light is turned on, the name of each product in the series is reflected, making it an excellent art piece,” Mr. Ibe said.

With new models and collaborations coming out every few weeks, G-Shock is a significant product for Casio, which reported net sales of 263.8 billion yen ($1.79 billion) for its 2022-23 fiscal year, which ended March 31. “Sixty percent of our whole sales are coming from watches,” Mr. Masuda said, “and 60 percent of that figure comes from G-Shock.”

During the past fiscal year, the company shipped more than 140 million G-Shocks; in 1983, it shipped 30,000.

Mr. Masuda said he still is surprised by G-Shock’s popularity: “I never expected this would happen.”

Even with flamboyant projects such as the gold watches, Mr. Masuda said, G-Shock was always and still is all about toughness. “I think the core fans were attracted by G-Shock’s toughness, strength and resilience, its core identity,” he said. “But we have to keep challenging ourselves and breaking the conventions. We are living in an age of sustainability and digitalization. You need resilience and toughness to survive.”

And what does the future of G-Shock look like? Mr. Ibe said, with a smile, “I think younger people will do that for us.”