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 and a half, but its value is still unclear. The valuation process could take months, Mr. Zimnisky said.