Actually the same stuff that coal and your pencil lead is made of - carbon, the atoms are just arranged in a way that make the bonds very strong.

The bonds are formed when carbon is at very high temperatures and under very high pressures. Natural diamonds are made deep down in the earth, about 180km below the surface, where high temperatures and pressures exist naturally. Under the earths crust is the mantel, which is made up of molten rock, metals and other materials. The temperature is very high at this depth - between 1100oC and 1400oC. The high pressures are produced by the weight of 180 km of rocks pressing down on it. Besides carbon, there are very small amounts of other substances, such as nitrogen and sulfur that can become trapped in the crystal when it is formed in the mantel. These impurities can give color to the diamond. One of the most rare is pink diamonds. The engagement ring that Ben Afleck gave to Jennifer Lopez is a pink diamond, making the ring worth $3 million dollars.

Diamond

Graphite

Carbon Chain Click on each of the molecules to see it from all sides. (Requires Chime plug-in)

Where does the carbon come from to make diamonds?

Most diamonds are made of carbon that was in the mantle since the earth was formed, but some are made of carbon from the bodies and shells of microorganisms like algae in ancient oceans. This organic carbon was buried in rocks that were dragged down into the mantle because of plate tectonics and continental drift. All living things on earth are based on carbon. If you or I were to somehow fall into an ocean trench at the edge of a tectonic plate where rocks are being dragged underneath a continent, we might reappear millions of years later as diamonds!