Krishna: Diamonds are made of carbon so they form as carbon atoms under a high temperature and pressure, they bond together to start growing crystals. Because of the temperature and pressure, under these conditions, carbon atoms will bond to each other in this very strong type of bonding where each carbon atom is bonded to four other carbon atoms. That's why a diamond is such a hard material because you have each carbon atom participating in four of these very strong covalent bonds that form between carbon atoms. So as a result you get this hard material.

 In the Earth's interior, the conditions are such that you've got some group of carbon atoms that are in close enough proximity that they start to bond. As other carbon atoms move into the vicinity they will attach on. That's the way any crystal grows. It's the process of atoms locking into place that produces this repeating network, this structure of carbon atoms, that eventually grows large enough that it produces crystals that we can see. Each of these crystals, each diamond, one carat diamond, represents literally billions and billions of carbon atoms that all had to lock into place to form this very orderly crystalline structure.

The carbon seems to have originated within the mantle of the Earth, so carbon that was already in the Earth. In other cases, there's evidence very curiously to suggest that the carbon may have originated near the surface of the Earth. The thinking there is that this carbon could have literally been carbon that was part of carbonate sediments or animals, plants, shells, whatever, that was carried down into the upper mantle of the Earth by the plate tectonics mechanism called subduction.

Diamonds are formed deep within the Earth about 100 miles or so below the surface in the upper mantle. Obviously in that part of the Earth it's very hot. There's a lot of pressure, the weight of the overlying rock bearing down, so that combination of high temperature and high pressure is what's necessary to grow diamond crystals in the Earth. As far as we know, all diamonds that formed in the Earth formed under those kinds of conditions

The diamonds that we see at the surface are ones then that are brought to the surface by a very deep-seated volcanic eruption.

 It's a very special kind of eruption  that occurred a long time ago in the Earth's history. We haven't seen such eruptions in recent times. They were probably at a time when the earth was hotter, and that's probably why those eruptions were more deeply rooted. These eruptions then carried the already-formed diamonds from the upper mantle to the surface of the Earth. When the eruption reached the surface it built up a mound of volcanic material that eventually cooled, and the diamonds are contained within that. These are the so-called Kimberlites that are typically the sources of many of the world's mined diamonds.

One of the things we know, therefore, about any diamonds that were brought to the surface is that the process of the Kimberlite eruption bringing the diamonds from the upper mantle to the surface of the Earth had to happen very quickly, because if they were traveling too long and too slowly they would have literally turned into graphite along the way. And so by moving quickly they essentially got locked into place into the diamond structure. Once the diamonds have been brought from high temperature to low temperature very quickly—and by quickly, we mean in a matter of hours—these eruptions, these Kimberlite pipes moving to the surface, may have been traveling at rates of 20 to 30 miles per hour. Once the diamonds are brought to the surface and cooled relatively quickly, those carbon atoms are locked into place and there's just not enough energy to now start rearranging them into graphite.

Krishna: 

Synthetic or lab-produced diamonds have been "grown" for many decades, and the process is exceptional now.

Every natural diamond is different, and can contain many impurities from within the earth, as well as signs of stress which may differ throughout the crystal.

However each synthetic diamond, made under more controlled conditions, is of uniform quality and any impurities can be more carefully managed.

What exactly are lab-grown diamonds and how do they compare with the real thing?

Synthetic diamonds are chemically and physically identical to naturally-occurring diamonds, and it can be very hard to distinguish them using the naked eye or even with a jeweler's glass.

Often the only way to definitively distinguish lab-grown from natural diamonds is by using specialist equipment that measures the molecular characteristics of a diamond such as absorption spectroscopy or photoluminescence.

Diamonds that are synthesized for jewelry can be colored by the introduction of certain elements including extra nitrogen for yellow, boron for blue or silicon for pink diamonds, or by using heat or irradiation treatments.

The two main ways to grow synthetic diamonds are the high-pressure, high-temperature method (HPHT) or by using chemical vapor deposition (CVD).

In the HPHT method, we place a small fragment of natural diamond to "seed" in a chamber filled with carbon and subject it to high pressure and high temperatures, in a fast-track version of the natural processes that take place over millions of years. The carbon then crystallizes around the seed, growing that original diamond by about a millimeter a day.

Then we chop a bit off the new diamond we created for the next seed, and repeat.

The CVD method involves heating a gas mixture of hydrocarbon and hydrogen in a vacuum chamber, causing the carbon atoms to separate from the gas and deposit as a layer of diamond onto a surface. The process can take several hours to create a thin layer of diamond, then repeated to create layers that are many millimeters thick.

Most natural diamonds are between one and three billion years old. They form mainly in old, cold cratons—ancient continental crust structures 150 kilometers or more below the Earth's surface—where temperatures reach 650 to 1,000°C and pressure is up to 100,000 greater than the earth's surface.

Atoms of carbon then get forced together under extreme pressures and temperatures and bond into crystals which slowly grow over millions of years. These unusual conditions mean that we only ever see diamonds when a volcano explodes, bringing them up to the earth's surface.

Most natural diamonds pre-date the first land plants, so diamonds are almost always formed from carbonate rocks rather than from compressed coal.

Lab-grown diamonds are typically much cheaper than naturally occurring diamonds, with prices up to 80% lower because they are not subject to supply constraints and the high mining costs of natural diamonds.

That's because a diamond can be grown from any source of carbon, and any organic material, when broken down into its core components. Researchers have made diamonds from all sorts of things—including tequila and even peanut butter.