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Formation from Carbon
The building blocks of diamonds are carbon atoms. While carbon can occur naturally in the form of coal from biological processes, it is not located in a geological region on the planet which has the required amounts of heat and pressure to form diamonds. The source of the carbon for diamond formation are pure carbon (known as "graphite") deposits, which accrued in the earth's mantle during planetary formation billions of years ago.
Heat and Pressure
Diamonds can only form at a depth in the earth's mantle of between 100 to 200 kilometers. This is the depth level where pressures of 5 to 6 gigapascals force carbon atoms into a crystalline structure. Diamond formation is also made possible because temperatures of 900 to 1400 degrees Celsius melt solid graphite into liquid carbon, facilitating molecular rearrangement.
Diamond Ores
Diamond crystals are carried from the mantle by volcanic magma to the surface of the earth. The magma then solidifies, with the diamond ore embedded in it. These carrot-shaped veins of ore-bearing volcanic rock are called "kimberlite." Diamonds can also be found in alluvial deposits, which were originally kimberlite that has been eroded and broken down by geological processes.
Synthetic Diamonds
Synthetic diamonds are manufactured in a laboratory by subjecting graphite to high pressure and heat in a sealed chamber. This mimics the geological process which produces natural diamonds in the earth's crust. The graphite turns molten, and carbon atoms re-form as crystalline diamond molecules. Another method to artificially produce diamonds is to superheat carbon into a plasma (also in a pressurized chamber) over minute diamond crystals. These seed diamonds accrue more carbon atoms and slowly grow.
Uses of Diamonds
Diamonds that are mined will eventually be used as either gems for jewelry, or for industrial applications. This is determined by the diamond's color, optical properties, size and type of external structure. Diamond crystals that are unsuitable as gems can be used for industrial purposes. These diamonds are utilized as abrasives for grinding, and as machine bits for drilling, cutting and polishing. Because diamond has excellent thermal conductivity, it is used to radiate heat away from highly sensitive electronic components. Diamonds are also used as a wear-resistant coating.