Natural diamonds are diamonds that form in nature, deep inside the Earth, rather than being made in a lab. Here’s a clear explanation: What are natural diamonds? • They are made of pure carbon. • They form about 150–200 km (90–125 miles) below the Earth’s surface. • They develop under extreme heat and pressure over 1 to 3 billion years. • Volcanic eruptions push them closer to the surface, where they can be mined.