Diamonds are the hardest naturally forming material on Earth, renowned for their beauty, strength, and durability. They have been known and hoarded by humans for thousands of years, and their name comes from the Greek word for invincible.
These stones come from deep within the Earth’s crust, formed there by the incredible pressure and relatively manageable temperatures. They form beneath the continental crust, from pure carbon. This carbon may either come from exclusively non-organic sources, organic sources, or a blend of the two. Diamonds formed from inorganic carbon are called harzburgitic diamonds, while those formed from some amount of organic carbon are called eclogitic diamonds.
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Diamonds form at depths of around 90 miles (150km), over millions and millions of years. The temperature at which they form is in the range of 2,000° Fahrenheit (1,100° Celsius) — too much hotter than this and the conditions are no longer suitable for their formation. Most are over a billion years old by the time they reach the Earth’s surface, with some diamonds being over three billion years in age — not much younger than the Earth itself.
The stones come to the surface when magma from far below the Earth’s surface begins coming up. Since they are found at such incredible depths — three to four times deeper than the depth at which a normal volcano originates — magma upsurges deep enough to bring them to the surface are relatively rare. Once this magma cools, it forms a rock known as kimberlite — or sometimes lamproite — which may be used as an indicator that diamonds may be found in that area.
By mining into one of these kimberlite dikes, or kimberlite pipes, diamonds may be uncovered. Of course, the presence of kimberlite does not necessarily guarantee that the precious stones will be present, simply that the volcanic upsurge originated at sufficient depth that diamonds could have formed. Often these kimberlite dikes will erode over time, and the gems will be carried away with the sediment, to accumulate in basins somewhere.
While kimberlite dikes are the most common place where diamonds can be found, others also exist. In some cases, glacial action may pick up diamonds and transport them many hundreds of miles, leaving them behind in their path or when they eventually melt. This has led to stones being found in locations that geologically were not appropriate, but not in sufficient quantities to make tracking glacial paths a viable method of hunting.
Extremely small diamonds may also be formed under certain extraordinary conditions. Such microdiamonds are sometimes formed, for example, when meteors strike the Earth’s surface. Although they are not of sufficient size to be particularly valuable, they do serve as a reliable indicator of impact craters from meteors.
For many centuries India was the world’s top source of diamonds, but eventually these sources were mostly depleted. In the modern world, nearly half of all those mined come from mines in southern and central Africa. The bulk of these mines are owned and operated by various companies of the De Beers Group, which is responsible for more than 40% of diamonds by value worldwide, and has held a virtual stranglehold on the world market since its formation in the 1860s. Large-scale mines also exist in Brazil, Australia, Siberia, and parts of Canada. Once mined, these diamonds travel the world to be cut and polished to create the beautiful gems we are all familiar with. Most cutting takes place in a few areas of the world, most notably New York, Antwerp, and Tel Aviv.