LONDON — High in the mountains of a small southern African kingdom, miners have unearthed one of the largest diamonds ever found, a glittering giant weighing 910 carats, or 6.4 ounces, and probably worth tens of millions of dollars.

The stone, found in the Letseng mine in Lesotho, is the fifth-largest gem-quality diamond ever found, and the largest in Lesotho’s history. It weighs more than a regulation baseball (about 5 ounces), and more than half as much as the average male human heart (about 10 ounces).

Gem Diamonds, the London-based company that owns a majority stake in the mine and that announced the find on Monday, described the diamond as being of extremely high quality.

It said the stone is a D color, Type IIa diamond, meaning it is colorless and very radiant, without any yellow tint or impurities like nitrogen that absorb light and decrease luster.