The diamond was so large that it obscured the face of Botswana’s president as he held it up for closer inspection on Thursday.

President Mokgweetsi Masisi grinned as he lifted the diamond, a 2,492-carat stone that is the biggest diamond unearthed in more than a century and the second-largest ever found, according to the Vancouver-based mining operator Lucara, which owns the mine where it was found.

This exceptional discovery could bring back the luster of the natural diamond mining industry, mining companies and experts say.

The diamond was discovered in the same relatively small mine in northeastern Botswana that has produced several of the largest such stones in living memory. Such gemstones typically surface as a result of volcanic activity.

“All of the stars aligned with that volcanic eruption, and the conditions were just perfect,” said Paul Zimnisky, an independent analyst in the diamond industry.

The rough diamond is large enough to fill an adult holder’s palm, and weighs more than a pound, but its value is still unclear. The valuation process could take months, Mr. Zimnisky said.

Still, the diamond will likely sell in the range of tens of millions of dollars, he added. The discovery is likely to be a boost not only for the diamond industry, but also Botswana, whose economy is heavily reliant on the export of diamonds.

“The big diamonds sell the small diamonds,” Mr. Zimnisky said.

Such whopping stones are no longer once-in-a-lifetime finds thanks to evolving technology. Lucara spotted an opportunity in Botswana when it dug up large quantities of small but coarse stones that looked like “chewed glass,” said William Lamb, the company’s chief executive. It was a hint that larger diamonds were probably being crushed in the retrieval process.

“A diamond is hard and you can’t scratch it, but it’s actually very easy to break,” Mr. Lamb said.

The company has made finding larger gems its objective, pushing for higher revenues over volume, Mr. Lamb said, holding up a resin copy of one his earlier trophies: a stone about a quarter the size of his business card.

A 1,109 carat diamond, unearthed at the same mine in 2016, was bought for $53m by London jeweller Laurence Graff, chairman of Graff Diamonds, in 2017.

Lucara has 100% ownership of the mine in Karowe.

Botswana's government has proposed a law that will ask companies, once granted a license to mine, to sell a 24% stake to local firms if the government does not exercise its option of becoming a shareholder, Reuters news agency reported last month.