Apart from the blue Emirates diamond, which are the other rare diamonds?

A rare blue diamond, worth $100 million, was displayed at an exhibition on of the world's rarest diamonds in the United Arab Emirates' capital, Abu Dhabi, on Tuesday.

Know about this rare blue Emirates diamond

The eight diamonds on display at the Sotheby's exhibition have a total weight of over 700 carats. They include red, yellow, pink and colourless diamonds.

Visitors focused on the 10-karat blue diamond from South Africa, considered one of the most important blue diamonds ever discovered. Sotheby's expects it to be auctioned off at $20 million in May.

Quig Bruning, the company's head of jewels in North America, Europe and the Middle East, said they chose Abu Dhabi for the current exhibition because of the Gulf nation's high interest in diamonds.

"We have great optimism about the region," he said. “We feel very strongly that this is the kind of place where you have both traders and collectors of diamonds of this importance and of this rarity.”

Rarest diamonds by colour that you may know about

Just like this blue Emirates diamond, there are other boldly coloured diamonds can be rare and hold great value, depending on the colour. ​​Coloured diamonds are graded in order to increase colour strength, from Faint, Very Light, Light, Fancy Light and Fancy to Fancy Intense, Fancy Vivid, Fancy Dark and Fancy Deep. The more valuable diamonds tend to fall within the Fancy Intense and Fancy Vivid grades.

The colour of a diamond depends on its elemental and structural makeup. They contain elements like nickel and nitrogen tend to be more yellow or brown in appearance. Since these elements are some of the most commonly found elements, these coloured diamonds are more common. On the other hand, diamonds composed of rarer elements or that are formed in peculiar environments and circumstances are rarer and therefore more valuable. Here's a breakdown of the rarest pieces that one may find.

Rarest of the reds

The trophy for “rarest diamond colour” goes to the red diamond. They are so rare, in fact, that once there was a 30-year gap (1957-1987) during which no pure red coloured diamonds were graded by the Gemological Institute of America (GIA).

Red coloured diamonds could be found at the now closed Argyle mine in Western Australia, where Earth’s shifting plate tectonics placed stress on diamonds over millions of years. When a piece of diamond experiences stress due to collisions like this, carbon atoms shift can shift within the diamond. If they shift just right, these newly relocated carbon atoms can cause light to be absorbed and transmitted differently than they would have pre-collision.

Right now, only three red diamonds with a weight of more than five carats exist, and most jewellery aficionados rightfully heap all the attention and praise on the 5.11 carat Moussaieff Red Diamond. But another renowned red diamond is the Kazanjian red Diamond, weighing in at 5.05 carats. It is an emerald-cut stone and is housed at the Smithsonian’s National Museum of Natural History. Its deep red colour caused it to be mistaken for a ruby for many years.

The beautiful blue pieces

Diamonds contain a chemical boron that can give the precious stone have a faint or deep blue colour. The more boron present, deeper the blue hue it will be. Blue diamonds form in places where one tectonic plate is pushed deeper in the Earth’s mantle by another, and this subduction brings boron to lower depths within the earth, providing the perfect ingredients and environment for a rare blue diamond to form.

The Hope Diamond, a whopping 45.52 carat blue diamond, currently calls the National Museum of Natural History its home. Containing boron, the piece has likely been extracted from the Kollur mine in Golconda, India, in thee 16th century. Its deep colouring includes a very slight violet element that cannot be seen by the naked eye. The Hope Diamond is infamous for an alleged "curse" that has followed its owners, with stories of bad luck and misfortune associated with those who owned or came into contact with the stone. Whether it is a fact or myth is debatable.

Gorgeous greens

Truly unique and stunning, green diamonds are also pretty rare. Most green diamonds are formed through the exposure of radiation underground, which are emitted by the decay of nearby elements such as uranium. This radiation results in carbon atoms within the diamond being “knocked out of place,” leaving vacancies or “empty pockets” within the diamond. These vacancies and the existence of other elements such as nitrogen, hydrogen and nickel (very rare!) result in blue and red light being absorbed into the diamond, leaving green light to be emitted and perceived by the human eye.

One among the rarest gems on the planet, weighing in at 41 carats, is the Dresden Green Diamond. The biggest natural green diamond ever discovered, is currently housed at The Green Vault Museum in Dresden, Germany. The diamond’s low nitrogen and boron levels result in an almost-flawless diamond, with the colour evenly distributed, increasing its rarity.

Opulent Orange

Like yellow diamonds, orange diamonds too are formed due to the presence of nitrogen. However, when the nitrogen is deeper inside a diamond (even if slightly!), light refracts differently, toward a more yellow-red blend, creating an orange appearance.

“The Orange", as it is called, is a fancy-looking, orange diamond, is a marvel of a rock, tipping the scales at 14.82 carats. It sold for $35.5 million in 2013. Another orange diamond, Pumpkin, is a 5.54 carat fancy vivid orange South African diamond, sold just one day before Halloween in 1997 for $1.3 million. It is estimated that only 0.05% of all-natural fancy colour diamonds are of the pure, unmodified orange variety.

Pretty Pinks

Pink and red diamonds are considered part of the same spectrum but are categorised differently due to the significant variation of colours. Pink diamonds have a weaker saturation, meaning their colour is lighter and softer. They can range from a delicate blush colour to a more vivid rose pink. Though pink diamonds are more commonly available than its red counterparts, they are still rare.

The Graff Pink Diamond is a gargantuan 24.78 carat, emerald-cut fancy intense pink diamond. The combination of sensational colouring and an uncommon cut (emerald cuts are typically associated with white diamonds) make this beautiful stone a rare one.

Interestingly, the diamond’s owner, Laurence Graff, had the diamond’s 25 natural flaws removed using modern technology; an astonishing accomplishment, considering one wrong move could have shattered the precious gem. The “renovations” to the diamond cost it 0.9 carats in weight, a reasonable trade-off to achieve near perfection.