Diamonds are made of carbon atoms that formed a bond due to the heat and pressure beneath the Earth's crust. The diamonds were then ejected from the Earth's mantle in a very fast deep-source eruption. They were locked into a diamond structure by the rapid cooling after the eruption.
All natural diamonds are formed in the mantle of the Earth. The mantle is the layer of magma beneath the Earth's crust. The heat and pressure in the mantle causes carbon atoms to form a crystalline pattern in which each carbon atom is bound to four other carbon atoms. An eruption beginning at the depths where the diamonds were formed then broke through the crust and scattered diamonds over the Earth's surface. This type of eruption was common when the planet was young but has not happened on Earth in millennia.
The eruptions that brought the diamonds to the surface moved material at rates of 20 to 30 miles per hour. The diamonds would have been transformed into graphite while traveling if the eruptions had been any slower. The diamonds and the surrounding lava had to have cooled in a matter of hours to retain the diamonds' crystalline shape and distinctive hardness.