This past February, a diamond known as “The Enigma” sold for $4.3 million. This massive gem is larger than a golf ball. But size isn’t the only unusual thing about The Enigma. It’s also a rare type of black diamond called a carbonado (kar-buh-NAH-doh). Carbonados are very mysterious. Scientists don’t know exactly what gives them their dark coloring or how they form.

Normally, diamonds form when the element carbon (C) is exposed to intense heat and pressure in Earth’s upper mantle. This superhot layer lies beneath Earth’s outer crust. Eventually, volcanic eruptions push the gems toward the planet’s surface. Unlike typical diamonds, carbonados are found in nonvolcanic areas in Earth’s crust. A few experts believe carbonados were brought to Earth on space rocks called meteorites. But most scientists think these diamonds were somehow made on Earth. For now, The Enigma’s origin remains a mystery.