Natural diamonds typically come from inside the Earth’s mantle. They form when carbon deposits are acted upon by a combination of high temperatures and pressure. Formation of a diamond from carbon requires one to 3.3 billion years, and they rise in magma to near the Earth’s surface through volcanic action. Once the magma has cooled into igneous rock, the diamonds can be mined.
The presence of carbon and magma is necessary for the formation of diamonds. Mined diamonds are stones that have reached locations near the earth’s surface. Primary sources of diamonds are volcanic pipes, while secondary sources include locations where erosion has occurred due to wind or water, freeing the diamonds from the lamproite or kimberlite matrix in which they exist. Diamond rates a 10 as the hardest known natural mineral on the Mohs scale, which rates minerals due to their tendency to scratch. In comparison, diamond is four times harder than corundum, the next hardest mineral on the scale.
Small impurities absorbed by diamonds as they form are responsible for the different diamond colors. Nitrogen contributes to yellow and brown colors in diamonds, while boron contributes to blue coloring and irradiation to green and perhaps red and pink coloring. Red diamonds are rarest, followed by purple, orange, pink, black, green, blue, colorless, brown and yellow. A natural diamond without impurities is colorless.