Diamonds are made out of carbon - highly organized carbon, that is. Geologists are still guessing how diamonds formed in the Earth from 1 billion to 3 billion years ago, according to a recent study in the journal Nature, but they think the recipe follows something like this: 1. Bury carbon dioxide 100 miles into Earth. 2. Heat to about 2,200 degrees Fahrenheit. 3. Squeeze under pressure of 725,000 pounds per square inch.