Diamonds are quite simply carbon atoms that crystallized and formed billions of years ago and predate the oldest known life on earth. Assembled in the most tightly bound crystalline configuration, a diamond is the hardest structure known to man; a diamond can only be scratched by another diamond.
Their formation requires very specific conditions involving the exposure of carbon-bearing materials to extremely high temperatures between 900-1300 degrees Celsius and enormous natural pressure. These conditions occur naturally in only one place on earth, in the layer known as the mantle, approximately 150-200km from the earth's surface where a combination of high pressures and temperatures exist.
Most diamonds are 3 billion years old, two thirds the age of the earth and are carried to the earth's surface from great depths in rare eruptions of molten rock or magma. This magma emanates though "pipes" joining deep cracks and fissures and erupts in small but violent volcanoes; these pipe formations are know as kimberlites. Within these pipe formations you often find volcanic rock, minerals, mantle fragments and sometimes embedded diamonds.
Diamond deposits are mined in different ways. Alluvial deposits are where diamonds have been removed from the primary source (Kimberlites) by natural erosive action over millions of years, and eventually deposited in an environment far away such as a riverbed, an ocean floor or a shoreline.
Marine diamonds are diamonds that have been transported downstream, but did not remain deposited on land, but made their way to the sea bed just offshore. Diamonds in marine areas are typically trapped in bedrock depressions such as gullies, potholes and channels.
Diamonds are currently mined in over twenty-five countries around the world on all continents except for Europe, Arctic and Antarctica. The most common places associated with diamond mining are South Africa and South America. Other countries such as Australia and Russia, are beginning to harvest more diamonds.
It is generally accepted that diamond deposits are found in the oldest parts of continents called "cratons", where the basement rocks are older than 1.5 billion years.
These cratons can be divided into two terranes: Archean-age archons, which are older than 2.5 billion years, and Proterozoic-age protons, which are 1.6-2.5 billion years old.