• A one-of-a-kind 1999 platinum Rolex Daytona, custom-made and never commercially sold, is hitting Sotheby’s auction on May 11 — and could fetch up to $1.7 million.
• Rolex didn’t start producing platinum Daytonas until 2013, making this early piece exceptionally rare. Only four were made for a private family commission, and this is the last one to hit the market.
• Rare watches like this are booming as investment assets, with global demand from younger and international buyers helping drive prices up more than 125% over the past decade.

A legendary timepiece is about to step into the spotlight.

A 1999 platinum Rolex Daytona is heading to auction on Sunday at Sotheby's Geneva, and could sell for up to $1.7 million.

The watch is made from platinum, a material Rolex did not use on Daytona models until 2013. Its face is mother-of-pearl, set with 10 diamonds. Unlike nearly every other Rolex on the market, it was not part of a standard collection. It was privately commissioned, custom-made for a client — something almost unheard of for Rolex.

"It's very unusual to come across a commission," said Pedro Reiser, senior watch specialist at Sotheby's. "There are other brands which might be more flexible and do these kinds of exercises, maybe on a regular basis — but not in the space of Rolex pieces where you barely can come across any commission whatsoever."

Only four of these watches are known to exist, made for the same family, each with a different dial. The watch heading to the auction block is the last one to be sold. The others have already gone for massive prices, topping $3 million.

There is big hype around this It is believed to have been created under the leadership of Patrick Heiniger, Rolex's CEO from 1992 to 2008. He ran the company during a time of major growth and secrecy and helped turn Rolex from a respected watch brand into a global luxury icon.

While rumors have swirled that Heiniger personally commissioned or wore a similar platinum Daytona, Reiser cautions that there is no confirmed link to this watch.

"That's more of a rumor," Reiser said. "Personally, I've never seen him with this piece, but I know that he used to love platinum watches — mainly Day-Date models. It's a nice story that accompanies the watch, but I think it's more of a myth."

The fact that Rolex made a platinum Daytona in 1999, long before it introduced platinum models publicly in 2013, is a major part of the watch's mystique.

"Back then, they only existed in stainless steel, yellow gold and white gold," Reiser said. "Having a platinum — the only known platinum Zenith Daytona — is very special."

This particular model stands apart even from its siblings.

"This is the only one that has a diamond-set dial," Reiser said. "The others had dark mother-of-pearl, lapis lazuli and turquoise stone dials, but no diamonds."

As more people, especially wealthy collectors and younger buyers, increasingly see rare watches as investments, the prices of these rare timepieces have climbed.

According to Knight Frank's latest index, watches have jumped more than 125% in value over the past decade, ranking them among the top-performing luxury investments, just behind rare whisky and high-end designer furniture. Even after a slight cooling, with prices rising only 1.7% over the past year, the five-year growth rate for watches of 52.7% signals the category remains a reliable long-term play.

Demand has only broadened, with more international buyers and a wave of under-30 collectors entering the market, Reiser said.