Carbon

• The story of diamond formation goes all the way back to the formation of Earth 4.5 billion years ago. Dying stars in the area left behind a cosmic "dust." The dust consisted of basic elements: Carbon, hydrogen, nitrogen, other gases and debris. This material orbited our sun and slowly condensed to form planets. As Earth formed, the material separated into three distinct layers; the crust, the mantel and the core. Diamonds form in the mantle, from carbon left after the planet's formation.
  
  

Mantle

• Pure carbon in its original form resembles coal. Only carbon that is exposed to the harsh conditions inside the mantle will form diamonds. The mantle, the middle layer of the Earth, begins beneath the crust at a depth of about 20 miles and is 1,800 miles thick. Diamonds form at depths of at least 90 kilometers, according to the American Museum of Natural History. The diamonds we find in the crust today were formed 2 billion to 3 billion years ago, according to the Public Broadcasting Service. They were carried to the surface by molten streams of rock.
  
  

Pressure

• It takes immense heat and pressure to turn a lump of coal into a diamond. The mantle is the only place on Earth where diamonds can form naturally. The carbon is heated to a temperature of 2,200 degrees Fahrenheit, according to PBS, and is subjected to great pressure. The carbon molecules rearrange themselves. They become denser and more organized and eventually turn into a diamond, the hardest substance known on Earth. Sometimes other elements become mixed in with the carbon during the process. This produces diamonds of all colors, including blue, red and rosy pink.
  
  

Pipes

• Pipes are long tubes of magma, or molten rock that extend from the mantle to the surface. On the surface they are volcanoes that erupt and spew lava. Diamonds can be carried along these pipes to the surface within the liquid rock; often they will cool embedded within the lava material.
  
  Molten rock from those depths carries indicator minerals in its composition. It is these indicators that prospectors look for when searching for diamonds. Kimberlite is a material found in diamond-rich magma, according to PBS. The more kimberlite or other indicating minerals found in a sample of land, the more likely that land is to hold diamonds as well.
  
  

Erosion

• Another way diamonds come to the surface is through erosion. Wind and water can eventually wear away kimberlite and expose diamonds trapped within it. These finds are valuable, but less desirable than finding a subsurface kimberlite pipe, which potentially contains many more diamonds.