When Feb. 14 approaches each year, the thoughts of many Americans in love turns to diamonds.

Nearly 20% of 2018 Valentines Day gift-givers plan to spend a total of $4.7 billion on jewelry, up 9.3% from 2017, National Retail Federation survey data show.

This handout photo from Gem Diamonds shows what's believed to be the world's 5th biggest diamond ever found, a 910-carat stone found in Lesotho's Letseng mine.
Gem Diamonds, via EPA-EFE

The day that might be or might not be named for a saint named Valentine falls during the four-month period from November through February dubbed "Proposal Season" by The Knot, an online site that helps couples plan weddings. February accounted for 7% of the engagements in The Knot's 2017 Real Weddings survey.

Valentine's Day: Planning to pop the question? If it doesn't work out, who keeps the ring?

Sticking with decades of recent U.S. tradition, 88% of the couples surveyed for the site's 2017 Jewelry & Engagement study opted for engagement rings with white diamonds as the center stone.

"They're far and away the most popular," said Shelley Brown, fashion and beauty editor for The Knot.

With that in mind, here’s a story that may inspire those who plan to give a loved one a diamond (or those who hope to get one). Like many stories, it begins both far away, and long ago.

Kingdom of Lesotho

It's summer now in Lesotho, a landlocked African nation of more than 1.9 million people and roughly 11,720 square miles that's surrounded by South Africa. Lesotho is located roughly 8,185 miles from Washington, D.C., 5,811 miles from London, and 8,537 miles from Tokyo.

Lesotho's mining employment has declined in recent years. Yet one active strip mine, owned by London-based Gem Diamonds and Lesotho's government in a respective 70%-to-30% partnership, has repeatedly set the global diamond industry abuzz in recent years.

Image from Gem Diamonds website shows a heavy truck at the company's Letseng diamond mine in Lesotho, Africa.
Gem Diamonds

Located at an elevation of roughly 9,900 feet in the Maloti Mountains of northern Lesotho, the Letšeng mine is the world's highest diamond mine. It is also the world's highest achieving U.S. dollar per carat diamond mine, according to Gem Diamonds.

In mid-January, the Letšeng mine produced a rare find: a 910-carat diamond, roughly equivalent to the size of two golf balls.

Even rarer, Gem Diamonds said the find is classified as a D-color Type II, diamond, the highest color and purest quality ratings.

Rarest of all, the Letšeng mine since 2006 has produced more than 60 gems of at least 100 carats, predominantly high-value, white diamonds.

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"There's no doubt that this is unusual for one mine, especially because it's such a low-yield production, said Tom Moses, executive vice-president of the Gemological Institute of America, an organization that's a global authority on diamonds, colored stones, and pearls. "It's a freak of nature."

The Letšeng Mine

Why have so many eye-popping diamonds been found here? It's difficult to determine with certainty, gem experts say.

However, Evan Smith, a Gemological Institute of America research scientist, has a promising theory. He's devoted his work to studying rare diamonds and characterizing their inclusions — the tiny imperfections typically found inside or on the surfaces of virtually all diamonds.

The GIA says diamonds are believed to have been formed "billions of years ago through a combination of tremendous pressure and temperatures of 1652–2372 degrees Fahrenheit at depths between 90 and 120 miles beneath Earth’s surface."

The stones later are pushed nearer the Earth’s surface by volcanic activity.

Photo shows a 102.34- carat flawless white diamond held by a model is displayed at Sotheby's auction house in London, Thursday, Feb. 8, 2018.
Alastair Grant, AP

But the diamonds recovered at the Letšeng mine are different, according to studies during the last two years by Smith and fellow researchers at the Carnegie Institue for Science and the University of Padua.

The very large stones contain tiny, iron-rich inclusions surrounded by a fluid jacket of methane and hydrogen, the researchers wrote in a peer-reviewed study recently published in Gem & Gemology, a GIA scientific journal. The metals typically are not found in other diamonds.

The findings indicate that the Letšeng mine's larger stones likely were formed roughly 224 miles to 466 miles below the earth's surface, deeper than the planet's shifting tectonic plates, Smith said in an interview.

"Those big diamonds are genetically different," said Smith. "They really do form a population of their own."

Additionally, Gem Diamonds and other gem-mining companies increasingly use x-ray like technology right at their excavation facilities. This helps locate large diamond stones within large blocks of ore and raw material.

"This is part of an effort to capture the diamonds before they're broken up into much smaller pieces" by the mines' mechanical crushers, said Michael Marty, the director of diamond supply management for Blue Nile, a Seattle-based online diamond jeweler and a global leader for engagement and wedding rings.

How much?

So, what's the likely sale price of a high-quality, 910-carat diamond?

Gem Diamonds declined to say. And several major diamond companies in the U.S. and Europe did not respond to USA TODAY messages asking the marketing question.

Marty, who estimates he sees tens-of-thousands of diamonds every year as part of his job, explained the silence likely stems from the way the world diamond market handles large and rare finds.

"You don't want to create a market for the stone until you've identified the right customer because there are so few customers for these types of items," he said. "You have to create a market before you put a price tag on the stone. It's kind of like a stock offering in some ways." 

Photo shows a model displaying a 110.92 carat round diamond during a 2017 preview at Sotheby's auction house in New York City.
JUSTIN LANE, EPA-EFE

Gem Diamonds plans to assess the 910-carat diamond at the company's Baobab manufacturing facility in Antwerp, Belgium, the global hub for rough, uncut diamonds.  There, the stone is weighed, examined and has its interior structure mapped.

"Ultimately, a 910-carat stone will likely produce multiple large, unique, interesting diamonds," said Marty. "The real art in the process is trying to figure out how do you get the best yield from the stone in order to maximize the value."

Unbound by any secrecy restrictions, Marty ventured that the diamond might sell for $50 million to $75 million — depending on prevailing market conditions, finding the right buyer and other variables.

He also estimated it could be cut to produce 10 to 30 stones of various sizes. Gem Diamonds previously said the 603-carat Lesotho Promise, found at the Letšeng mine in 2006, sold for $12.4 million and was crafted into 26 flawless, smaller diamonds.

Perhaps a somewhat more affordable offspring of the 910-carat diamond will be on the market come next year's Valentine's Day.

Follow USA TODAY reporter Kevin McCoy on Twitter: @kmccoynyc