Blue Diamonds from the Ocean Blue
Blue diamonds have long been a geological enigma. The element boron, which causes their color, is extremely scarce in the mantle where diamonds grow. So how did these blue diamonds become colored by it? Research published by GIA scientist Evan Smith in the journal Nature reveals that blue diamonds likely formed deep in Earth's transition zone (410–660 km), reaching into the lower mantle (>660 km). This is approximately four times deeper than most other kinds of diamonds, which form near the base of old and thick continents at depths of about 150–200 km. From examinations of blue diamonds' inclusions, Smith concluded that boron was likely brought into these deep areas by the subduction of oceanic plates beneath the lower mantle.