Natural diamonds are found at depths of more than 130 kilometers and are usually formed between 2.4 and 3.2 billion years ago, but can also be as young as 60 million years old. Diamonds have a unique cubic crystal structure. Each carbon atom in a diamond is tightly bound to four other carbon atoms, making it the hardest natural substance known in the world. Diamond also has high thermal conductivity, chemical resistance, and high transparency. Diamonds are formed when the earth is at 900-1300 degrees Celsius and at an atmospheric pressure between 45,000 and 60,000 degrees Celsius, and when the carbon-containing environment below the earth’s surface meets these conditions. If the stratum is stable, the temperature will not be too high at this pressure, which is favorable for the production of diamonds. Diamonds may be deposited if the sedimentary rocks contain methane or if calcium carbonate decomposes under these conditions, and these diamonds are sometimes carried to the surface by uplifted alkaline lava. Lava often contains large amounts of gases (e.g., carbon dioxide and water vapor), so it can move very quickly (10-30 km/hr) and may even rise close to the speed of sound as it approaches the surface.
Diamonds may turn to graphite when the pressure drops, but if the lava cools quickly enough at the surface, the diamonds are “frozen” and remain. When lava is close to the surface, it expands as the pressure decreases, forming flared columns with large tops and small bottoms. Minerals with diamondiferous columns are unusual, such as garnet, which is produced only at high pressures and often has high amounts of chromium (Cr-Pyrope) or titanium (Ti-Pyrope), and which is known as a brecciated mica-peridotite (Kimberlite or Lamproite). The surface has weathered to a yellow ground, but deeper down it is a pristine blue ground. kimberlite originates only from the depths of the ancient, stable Continental Shield. The lower temperatures under the Continental Shield can be torn apart when the Mantle convects , at which point the deeper lava rises rapidly and brings the diamonds from the floor to the surface. If the lava cools too slowly, the diamonds will turn to graphite as a result of the high temperatures at low pressure. If the lava is small, it cools rapidly and can freeze into diamond-bearing columns. Outcroppings of this special deep-rock column can only be seen in certain areas of the world.
After weathering the Kimberlite outcrops, the diamonds were washed away by rainwater and deposited in the riverbed, and after flash floods these diamonds drifted to the mouth of the river where they were mixed with the sediment along the banks. The continents of the earth were originally one big piece, and at that time, India was situated immediately to the east of South Africa, while Brazil was situated to the west of South Africa. Kimberlite diamonds from South Africa have been found in the depressions of India (such as the present Deccan Plateau) and the plains of Brazil (such as the present Jequitinhonha River). One hundred million years ago the continents began to drift apart, and many diamonds were carried by the coastal migration of the sea to the beaches along the coast (e.g., Southwest Africa). The first diamonds to be discovered were these drift diamonds. Diamonds from India and Brazil were discovered long ago, but they actually originated in South Africa. In South Africa, diamonds in riverbeds are guided by rock pillars that can be traced back to their source.
Rare diamonds are found in two types of igneous rocks, peridotite and eclogite, both of which are found 120-200 kilometers below the earth’s surface in layers of the earth’s crust and continue to release the carbon that makes up diamonds. The diamonds that are produced are carried to the surface at the right time, by kimberlite and lamproite, by uplift. These are the two rocks that diamond prospectors use to find diamonds. Natural diamonds are mainly sourced from the mother lode of Kimberlite, but the amount of diamonds contained within them is very rare, and it usually takes crushing several tons of ore to find a 1 carat (1/5 gram) diamond. Most natural diamonds are of poor quality and can only be used as super-abrasives for industrial purposes. The raw diamonds that are ground into gemstones only account for about 10% of the total natural diamond production.
