How Diamonds Form

Diamonds go through a lot before you see them in the jewelry store.  Diamonds are actually formed deep in the earth under extreme heat and pressure then they are transported upward through violent changes until they reach the surface.  After that it is cut and polished to reveal the beauty you see in the store.

Extreme temperature and pressure changes in the earth cause dramatic changes in many substances and they are most powerful when they work together.  The combining of the forces is what causes the atoms of a diamond to link together into a particular form.  This unique bonding is what makes the diamond the hardest substance on the earth.

Diamonds are made of carbon.  Graphite is also made only of carbon.  However, their individual properties are very different.  Graphite atoms are arranged in bonded layers with weak bonds between them, but diamond atoms are arranged in tight three-dimensional patterns bonding them strongly in all directions.

Diamonds are formed in a temperature range of 1652 to 2372 degrees Farenheit and between 45 and 60 kilobars of pressure.  You can imagine the difference in temperature and the kilobar pressure is roughly 45,000 times that of the pressure at sea level.  This combination occurs between 90 and 120  miles beneath the surface.  Diamonds are formed in igneous rocks inside cratons which are located only under continental land masses.

Diamonds can remain underground for hundreds of millions of years, but delivery to the surface is a rapid process.  Diamonds are deposited in two types of rock: kimberlite and lamproite and these deposits carry the rocks the diamonds formed in to the earth's surface.  Diamonds are delivered to the surface by a series of gaseous explosions, similar to popping the cork on a champagne bottle.

Sounds like we should have diamonds lying around all over the place right?  The fact is that kimberlites themselves are widespread - about 6,000 were known worldwide in the 1990s.  However fewer than 1,000 of those actually contained diamonds and only about 50 had enough diamonds to be mined.  Of those 50 only about 20 are still being mined today.  Thinking in those terms it is understandable that gem-quality diamonds are rarer than you think as you look at the arrangements in your jeweler's case.