Humans have been fascinated with gemstones for thousands of years. They are among the rarest and most desirable minerals on the Earth. Throughout history, gems play essential roles in economics, politics, religion, law, and fashion.
We collect and possess gems for many reasons: to adorn ourselves, for our surroundings, to demonstrate wealth, as investments, or to benefit from their supposed healing properties.
Gemmologists, those trained in the science of gem materials, are like detectives. They identify gemstones and determine their nature, whether they are natural, treated, or synthetic. Of all the gemstones, a diamond's properties are unique and among the most highly prized, valued, and widely studied minerals.
Just carbon - A diamond is the three-dimensional form of carbon, the flatform is graphite, and sheets of it can come off, which is how a pencil makes a mark on a paper. However, when carbon is formed into a three-dimensional shape, it gets high hardness, high strength, and a high refraction index. And much beauty of the light inside and the ability to be transparent.
Geological Origin of Diamonds
Our planet structure comprises the crust, the mantle, and the core. The crust is essentially the Earth's surface, where we live and where all our gemstones, including diamonds, are found.
The crust is the thinnest part of the Earth's regions at around 40 kilometers (25 miles) thick. The thickness will vary depending on the landmass's age and whether it is under the oceanic crust or the continental crust.
To put this into perspective, the whole Earth is 12,742 km (7917.5 mi) in diameter. The crust is divided into zones, known as tectonic plates, continually moving over the planet's surface. They're pulling apart, pushing together, and sliding under one another, forming geological processes such as earthquakes and volcanoes.
It is the volcanic eruptions that brought diamonds to the surface of the crust millions of years ago. The mantle, which is just beneath the crust, consists of mainly molten rock, and it is here where diamonds form. The core is at the center of the planet and has two areas: the inner core, which is mostly solid, and the outer core, mainly molten.
The place where diamonds form is known as the geological origin of diamonds. There is a range of temperatures and pressures in the mantle, and diamonds mainly form under pressures of 50 kilobars at depths of 150 kilometers (94 miles) on average. A kilobar is 1,000 times atmospheric pressure, so the pressures that diamonds format are 50,000 times that of atmospheric pressure. The temperature increases the further down into the mantle we go.
However, this temperature varies depending on where you are in the mantle in relation to the crust. Below the thinner oceanic crust, at 150 kilometers (94 miles), the temperature is around 1,500 degrees Celsius. Underneath the thicker area of the continental crust, known as a craton, the temperature is slightly lower at approximately 1,100 degrees centigrade.
Under these cratons, at slightly lower temperatures, diamonds have the right environment in which to form. So, having taken into account the Earth's structure, it is clear we now have a recipe for creating diamonds.
One of the reasons that diamonds are so rare is that they require such specific circumstances to form. They need a temperature between 1000 to 1300 degrees and a pressure of between 45 to 60 kilobars. These conditions are found at depths of 135-180 kilometers within the mantle and only found under cratons. They also need a carbon-rich environment.
Without carbon, diamonds would not form as predominantly they are made of that single element. Meeting these conditions provides an ideal environment for the formation of diamonds. If only one of these criteria is not met, the diamond will not form. The carbon will bond differently or with other elements and form different minerals.
In the upper mantle, carbon is believed to make up only a tiny fraction of 1% of this layer. Still, additional carbon is recycled continuously from Earth's surface due to the oceanic crust's subduction (when one tectonic plate moves under another.)
Cratons are the oldest, thickest areas of the crust. The word comes from the Greek Kratos, meaning strength. These areas have been folded, heated, stretched, and crushed by the Earth's geological activities for millions of years.
Formed far back in geological time cratons we know, through scientific research and analysis, some are over 2 billion years old. They are now left as thick, tough, rigid, and stable landmasses unaffected by current geological movement.
The pressure and temperature conditions required for the formation of diamonds are under the oldest cratons, the archons, and the protons. Look at where the cratons are formed on a map. The cratons coincide with the most critical diamond-producing countries such as South Africa, Botswana, Australia, Canada, and Russia.
While North America has the most massive known craton globally, there has never been a profitable diamond mine in North America.
How are diamonds found?
Diamonds come to Earth’s surface from the mantle, where a specific volcanic eruption type forms them. This eruption, called a “kimberlite eruption,” spews out Kimberlite, an igneous rock containing diamonds in its rock matrix. The last eruption was over 100 million years ago, so these gemstones have been waiting a long time to be found.
Kimberlite eruptions occur within continental cratons with the environments to help diamonds maintain stability on their journey to the surface. Knowing this helps diamond exploration by narrowing down the geography where Kimberlite eruptions occur. Geologists and diamond prospectors look for kimberlites, pipe-like formations created by the eruptions.
How are Kimberlite pipes minded? When found, a kimberlite pipe is tested. If the concentration of diamonds is high enough to allow economic extraction, massive quantities of earth will be dug out from an open pit’s surface. Huge trucks and giant steam shovels dig a deep hole. The deepest open-pit diamond mine currently, The Jwaneng diamond mine in Botswana, is 400 meters (1300 feet) deep.
Another geological source of diamonds is in alluvial or placer deposits. Diamonds in the kimberlite host rock are exposed as the stone is washed away by streams and rivers over millions of years.
The diamonds freed from the rock are deposited in the stream sands, a river bed, an ocean floor, or a shoreline. Alluvial deposits generally have a higher percentage of gem-quality diamonds than primary deposits (diamonds form deep within the Earth).