Diamonds and graphite or the "lead" in your pencil are made from the same element CARBON. Now just like H₂0 which can exist in different forms (ice, liquid or steam) so can the element carbon. At low pressure the stable form is graphite ...its basically what makes up the charcoal briquettes that you use in a BBQ... its carbon... coal is the same plain old simple carbon. BUT at high PRESSURE which occurs at depths greater than about 100 miles DOWN, graphite o is NOT stable. just like ice is not stable at room temperatures, the effects of pressure are to make a new arrangement of carbon atoms stable. This new arrangement of carbon atoms is called DIAMOND.

Diamonds are formed deep in the earth and sometimes are rapidly brought up to the surface (so fast that the diamond can not revert back to its stable form at low pressure) in magmas. These particular magmas are called kimberlites. There are some in South Africa, there are some in Australia there are some in Wyoming in the USA and many other places around the world.

Kimberlite is a kind of volcanic eruption that comes up extremely violently through the crust from an unknown depth, possibly ejecting at multiple kilometers per second when it reaches the surface. The minerals that make up kimberlites are very rich in iron and magnesium, probably derived from the Earth's mantle. The thing that makes Kimberlite different from ordinary lava is the heat and fantastic pressures that result from the violence of its formation. What would be other forms of carbon, either becomes diamond in the ejecta, or the eruptions originate deep enough in the mantle that diamond is chemically stable, and so can be brought near the surface. Color in diamonds results from impurities in the crystal matrix.

I am not aware of any Kimberlite eruptions to have occurred within the last 100 million years. I don't think anybody really knows what causes them, or how devastating they are to the Earth's surface when they do happen: violent as they may be, a single Kimberlite eruption probably doesn't have enough mass to seriously alter the climate, but they may come up in swarms.

Diamonds in Kimberlite almost certainly have nothing to do with coal; they represent a carbon source that is probably much older than the coal swamps of 300-400 million years ago.

:-) Yes, in fact, if you drop a hot diamond into liquid oxygen, it will burn!

Natural diamonds form by taking pure carbon and holding it under extremely high pressure and high temperature for a very long time (hundreds of thousands of years). Diamond is one type of crystal, one which is made from pure or nearly pure carbon. They are only formed deep underground, where pressures and temperatures are high. For diamonds to reach the surface of the earth--or even close enough to be reached by mining--they must be carried up by some process. One type of process is a kind of underground volcano which brings deep rocks up to the surface without actually breaking the surface. Kimberlite is one type of rock which has been explosively thrown up from deep in the Earth up into the shallow crust. Kimberlites are carrot-shaped, hard rocks, like an upside-down icicle pointing upward. Kimberlites bring other rocks and minerals from deep in the crust to the surface, which is why they are good sources for finding diamonds.

Unfortunately, many diamonds sold today come from parts of the world where the diamonds are the result of slavery and war, which has earned the nickname "blood diamonds." Many people are choosing to buy other gems to avoid having their money go to such slave owners and gangsters. Some scientists think we will be able to make artificial diamonds which are even prettier (more pure) than natural ones, so then we won't have this concern.

Diamonds are made of pure carbon (like the graphite in a pencil). It doesn't look anything like carbon because it's crystal structure changed when the carbon was at high temperatures and pressures (~100 km below the surface of the earth, and ~700 deg C). An analogy is how water changes its form when it freezes--at specific pressures and temperatures, water transforms to ice--at certain pressures and temperatures, carbon transforms to diamond. A Kimberlite is a "pipe" of high pressure minerals that explode to the surface. Diamonds are the primary mineral in kimberlites. Kimberlites are naturally occurring--they are not man made--and geologists and mineral surveyors use them to gain access to minerals that they otherwise wouldn't have access to (we can't drill 100 km below the surface of the earth). Diamonds are technically unstable at the surface of the Earth, but because the reaction rates are slow, they are metastable, meaning that a diamond wont suddenly change back to a lump of carbon. Kimberlites are generally not related to plate margins and are usually located within the centers of plates. Colors in diamonds are attributed to elements such as copper, iron,cobalt, etc. In terms of whether or not diamonds are a girl's best friend,consider that diamond is the hardest mineral we know of and is constantly being fought over (check out the movie Blood Diamond to "learn" a bit more about the controversy surrounding diamond mining). Diamonds may not be forever, but they will certainly last your entire lifetime--as will all minerals and gems.