Diamonds are made of the element known as carbon. The same as the carbon in your body or graphite or Talc even. The difference is in the degree of concentration. The more concentrated the element the closer packed its molecular structure and the denser it becomes.
To make a diamond requires a minimum pressure of 435,113 pounds per square inch (psi or 30 kilobars) with a temperature of 400 degrees Fahrenheit (205 Celsius). Most diamonds are thus formed around 100 miles (160 Kilometers) below the earth's surface. Where such conditions exist.
Diamonds eventually make their way to the surface in what are known as Kimberlite pipes. These are basically channels of molten lava rising like an elevator through the earths mantle and crust. The magma cools over time leaving the conical veins of kimberlite rock that contains the diamonds. Kimberlite is a bluish rock that diamond miners look for when seeking out new diamond deposits. Many of these are around and they are names after the Kimberley region of South Africa where they are commonly found. Most of these eruptions occurred around 1,100 million and up to 20 million years ago.
Diamonds are often found in river beds where they have become dislodged from the kimberlite pipes and then rain or glaciers might move the diamonds over thousands of kilometers or miles from their original location. Diamonds can be found in Australia, Borneo, Brazil, Russia and many African countries, including South Africa and Zaire.
Diamonds are found as rough dull looking stones and require polishing and finishing to resemble the diamonds that we know so well.