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.eologists 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.