Smithsonian Magazine has an article about the discovery of a very large diamond.  A 2,492 carat diamond is the largest found since 1905.  It was discovered this week in the Karowe mine, about 300 miles north of Botswana’s capital city of Gaborone.  That mine has produced four other large diamonds in the last decade.

This week’s find is almost the largest diamond uncovered by miners in history, second only to the 3,106-carat Cullinan diamond found in South Africa in 1905. That gem was cut into stones and put into the English Crown Jewels. An even bigger, black diamond was found in Brazil in the late 1800s, but experts think that one came from a meteorite.

Diamonds in Botswana are found in volcanic features called kimberlite pipes. These carrot-shaped structures brought rocks that formed deep within Earth’s mantle up to the surface during volcanic eruptions long ago. The heat and pressure that rocks experience at profound depths, between 93 and 280 miles underground, are critical to forming diamonds.