It is rare to find photographs of ‘rough diamonds’ of a fine jewelry quality on the internet. Rough diamonds are stones in their natural state as they are found in the earth before being cut and polished to become the jewels that you know. Rubel & Ménasché is proud to present an exceptional photo to you. We asked one of our gemmologists to analyze this gem for us.
Diamonds form deep in the Earth’s mantle at depths of more than 800 km.
Most of the diamonds originate from the lithosphere, nestling 150 – 200 km below the Earth’s surface.
However, some ‘superdeep diamonds’ are formed deep within the Earth, as deep as 800 km under our feet. These are still shrouded in mystery for scientists.
Extreme formation conditions
Diamonds are formed under extreme high pressures and temperatures.
This makes diamonds one of the hardest materials on earth, at least according to the Mohs scale. They need around 75 tonnes/cm2 of pressure, or the equivalent of a human being subjected to the pressure of 15 Eiffel Towers. While temperatures must reach around 1300°C
Once these conditions have been met, the only essential thing missing for our diamond to crystalize is carbon! As without it, our diamond would not exist. The carbon atoms shrink considerably under crushing pressure, which is why diamonds are extremely hard. Without such high pressures, our diamond would merely be graphite, like the graphite in our pencil leads…
Once it has crystallized, a long magmatic and geological process takes place. Millions, if not billions of years are needed before the diamonds reach the earth’s surface, transported mainly by Kimberlite rock. This rock also lent its name to the famous Kimberley mine in South Africa, opened in 1871.
Primary and secondary deposits
Experts talk of ‘perfect crystallization’ when rough diamonds take on an octahedral shape. This is a typical shape for a rough diamond, generally associated with primary deposits: where diamonds are mined directly in magmatic rock, as in the case of the Jwaneng mine in Botswana.
Over time, erosion affects the rocks, leading some diamonds to end up being washed down the mountainsides and into the rivers. We then talk about secondary deposits.
In this case, most of the diamonds will have more random shapes, further removed from perfect octahedrons. Congo diamonds are a good example of secondary deposits.
Finally, some diamonds are found in the ocean depths. The best-known undersea deposit
is off the coast of Namibia. Mining here is costly, but profitable, since such deposits contain pink diamonds, now extremely rare since the Argyle mines in Australia closed down.
Examining rough diamonds
Certain triangular shapes, known as ‘trigons,’ can be found on rough diamonds such as the one shown below. These bear witness to the extreme conditions under which the diamonds were formed. They are essential when distinguishing rough natural diamonds from other potential gems.
Trigons resemble engravings on the surface of diamonds. Once the rough has been cut and polished, the trigons disappear, giving way to the adamantine brilliance we know.
Using different methods, scientists are now able to determine the age of rough diamonds. The first is carbon dating: by comparing the ratio between the carbon isotopes 12 and 13 which are trapped within the diamonds, scientists are able to tell what elements they have been formed from, and then estimate their age. Inclusions also play a key role in dating diamonds. By analyzing them, experts can retrace the diamonds’ life and deduce the temperature and pressure that existed when they were formed. A diamond is a mine of information. The oldest diamonds studied by scientists date back 3.5 billion years and were found in the Diavik and Ekati mines in Canada.
Nathan Bensoussan, Gemmologist at Rubel & Ménasché
I have always been fascinated by minerals and gems, so I knew very early on that I wanted to be a gemmologist. There is a saying in the industry, which I find to be very true: “If you can see into a diamond, then you can see into any stone.” Diamonds are extremely specific, technical and demanding stones. Apprenticeships at Rubel & Ménasché enable you to achieve a very high level of perfection when working on diamonds, something you can’t get at any other school. This is the strength of the Maison’s know-how.
As a quality controller, I am responsible for ensuring that the batches which pass through my hands meet our clients’ specifications. Each client has their own specific requirements. We use the 4Cs of diamonds to assess the criteria for the stones. I will therefore check the fluorescence of the gems using an ultraviolet box, check their color using a simple white card, examine the inclusions and the cut, by using a sortoscope (a pair of binoculars that magnifies diamonds 10 times).
My days are quite varied. I can sort stones from 1 carat to less than 2mm in diameter (0.03ct).
Not to mention diamonds with round, oval and pear-shaped cuts…
The diversity of the fancy diamonds we can have at Rubel & Ménasché is almost infinite. This trains our eyes for all kinds of shapes.
We see some very beautiful stones daily, sometimes exceptional specimens…
I will always remember the day that I held a rough diamond of more than 50 cts in my hands…
I was shaking to see such a marvel of nature!”
