Carbon + Pressure + Temperature + Time = Diamond
That diamond in your ring was a lump of carbon that underwent compression under a pressure of 45 to 60 kilobars, at a temperature of 900 to 1300 °C, about 140 to 190 kilometers under our planet's surface, for about 1 billion to 3.3 billion years, to become the beautiful, rare gem that is the symbol of love.
Rarity equals high value. What could be rarer than a diamond, a piece of crystallized carbon, with the most unique optical dispersion properties, forged in the heart of the Earth, for billions of years? The global diamond mining industry has capitalized on this rarity value of the carbon allotrope, which also happens to be the hardest and most thermally conductive material in the world. According to Bain & Company's world diamond report, retail sales of diamond jewelry grew to USD 72.1 billion in 2012.

The top diamond producing countries of the world are Botswana, Russia, Canada, South Africa, Namibia, Angola, Australia, Congo, and Zimbabwe. Their combined production in 2012 was 128 million carats. In 2011, US consumers bought diamonds worth USD 22 billion. According to a report by GIA, from the 1870s, more than 4,500,000,000 carats (900,000 kg) of diamonds have been mined by now. The rising demand for the gem powers the diamond mining industry, which employs various techniques to mine its ores from the depths of our planet. Here's a brief overview of diamond geology and mining techniques, employed by companies like De Beers, Alrosa, Rio Tinto, and Petra.
Diamond Geology
First of all, the dispelling of a romantic notion. Diamonds aren't formed from coal. Most of them are forged from carbon that's trapped inside the Earth's mantle, at the very beginning of our planet's formation (about 3 to 4 billion years ago), at a depth of more than a 100 kilometers, below the Earth's surface. Whereas coal is a sedimentary rock, formed from organic plant matter, some millions of years ago, which is usually buried about 2 miles below the planetary surface.
Diamonds are allotropes of carbon, formed about 150 kilometers below the planetary surface, under the effect of 45 to 60 kilobars of pressure and an average temperature of 1050° C. Every single diamond is about a few million or 1 to 3 billion years old.
As our molten planet cooled down and formed a crust, volcanic activity and magma pushed these diamonds to the surface, through volcanic outbursts. In time, these tunnels of magma flow, just below the surface, solidified to form rocks. Hardened volcanic pipes got formed (known as Kimberlite and Lamproite pipes), which encased millions of diamonds. These are the sites, where the biggest diamond mines in the world are situated.

Some diamonds that made their way to the surface through volcanic outbursts, got distributed over ocean floors and riverbeds through soil erosion. Diamonds are also found in huge amounts, inside extinct volcanoes and there are chances of finding them in the live ones also but that obviously is a difficult task.

Some diamonds are also formed in subduction zones (where a layer of the Earth's crust is pushed below another) and at meteorite impact sites. A few rare diamonds are formed in space and deposited on the planetary surface through the fall of meteorites.
Diamond Mining Methods
Mining processes vary, according to the nature and the structure of the diamond ore and also the position from where it has to be extracted. It may be in a secondary deposit like a riverbed or buried deep in a solidified volcanic pipe. Depending on these factors, there are four main processes that are employed, which are described in the following lines.
Open Pit Mining
Open pit mining is extracting diamond ores from solidified Kimberlite deposits, that are very close to the surface. This technique is used, when the underlying deposit is covered by a shallow layer of sand, gravel, or easily penetrable rock. A tunnel is drilled to access the deposit. The mines can be easily identified due to their circular shape and can be easily spotted in satellite images, due to their large sizes.
open pit mining
The now closed Mir mine in Mirny, Eastern Siberia, in Russia is the deepest open pit diamond mine in the world, with a depth of 525 meters and a diameter extending up to 1,200 m. The Jwaneng mine, located in the Kalahari desert, in south-central Botswana, is the richest diamond mine in the world, and is owned by Debswana (a partnership between De Beers and the Botswana government).
Alluvial Mining
There are certain secondary deposits of diamonds which are created by erosion of top soil layer, from volcanic sites. Over time, these diamond ores get deposited and embedded in the seabed and riverbed. To extract the diamonds, a sand breaker or artificial dam is created, which holds the water back. Then the sea or river bed is manually or mechanically dug and the ores are sent to a refinery for extraction.
alluvial mining
About 24% of the world's diamond supply is sourced by alluvial mining. Deposits are known to exist in Sierra Leone, Democratic Republic of Congo, Ghana, Cote d'Ivoire, Central African Republic, Guinea, Liberia, Tanzania, Angola, Togo, Brazil, Venezuela, Guyana, and South Africa.
Marine Mining
To extract diamond ore, that is embedded deep in the sea bed (marine littoral deposits), marine mining operations are carried out. Diamonds transported by rivers and deposited on the sea bed get buried over time. For example, the Orange river flowing through South Africa, has deposited diamonds on the Atlantic coast, for millions of years. In this type of mining, extraction involves drilling the sea floor to depths of 90 to 140 meters, to reach diamondiferous gravel.
marine mining
Special sea vessels with drilling, extraction, and processing facilities are built, that carry out this operation. The sea floor is probed for diamond deposits by specialized seabed crawlers, which employ hoses to lift up pay dirt. The seabed is also drilled, to access diamond rich gravel. The extracted gravel is airlifted to the coast for further processing. De Beers is the leader in global operations of this kind, with operational vessels off the coast of Namibia.
Underground Mining
As an open pit mine is dug and diamond ore is extracted, its depth goes on increasing, until a point is reached, when continuing operations underground, becomes a logistical necessity. Generally, companies only consider going underground, if the mine continues to yield diamond output at the deeper levels.
underground mining
Also, in some cases, the shape of a Kimberlite deposit makes it inevitable that underground mining be employed as the extraction method. When deposits are found to be horizontally spread, deeper below the ground, underground mining is the only option. De Beers Canada's Snap Lake mine, DBCM's Venetia Mine in South Africa, and Debswana's Jwaneng Mine in Botswana are some examples.
After being extracted by any of the mining methods described above, the diamondiferous rock is crushed, to extract manageable chunks from it. Next, it is mixed with ferrosilicon slurry and through a technique called Dense Medium Separation (DMS). Diamond-rich concentrate is separated from it, using centrifugal techniques. At this stage, the concentrate is crushed even more, to extract diamonds. Using X-rays, laser sorters, and other techniques that exploit the properties of diamonds, they are finally separated from the concentrate, to be cut and polished.

Diamond mining is hard work and companies have to scan large areas and dig deep, for any chances of successful detection. The processes use colossal extraction machinery, which has to be customized and built according to the topography of the mining site. The mining juggernaut will continue drilling and extracting the precious stone, until none are left hiding under the planet's crust.