Blue-Green Wonder is Beyond Rare

Blue diamonds are rare, green diamonds are rarer still. But diamonds with both a blue and a green hue are virtually unheard of.

That's why the upcoming auction of the 5.50-carat fancy vivid blue-green Ocean Dream at Christie's Geneva next month is causing such excitement.

Diamonds derive their blue color when a small number of carbon atoms are replaced by boron during their formation deep in the Earth's mantle. That already makes them extremely rare.

But if, after it has formed, the diamond is then exposed to natural radiation near the Earth's surface - for example, from radioactive minerals in the surrounding rock - then the outer layer of the stone can develop a green hue. 

The odds of both things happening to any given diamond - forming blue and then gaining a green hue - are so remote that there are only two named examples known to exist in the world.

One of them is the Ocean Dream, which is described as blue-green because the blue hue dominates.

It was cut from an 11.70-carat Type IIa rough recovered in central Africa in 2002, and it carries an $8.75 million to $12.8 million estimate (CHF 7 million to CHF 10 million).

Its only known rival, the 1.6-carat Ocean Paradise, was also formed blue, then gained a green hue, but it is graded as green-blue, because the green dominates.

It is a much smaller stone the Ocean Dream and the color is less intense (fancy, rather than fancy vivid) and more pastel‑leaning.

It was cut from a 6.43-carat rough, is owned by the Nahshonov Group and has been valued at $2.67 million.

The Ocean Paradise was discovered in 2012, in the historic diamond‑producing region of Diamantina, in the state of Minas Gerais, Brazil.

There is no public information available on the exact origin of the far larger Ocean Dream, just that it was recovered from a country in Central Africa.

What we do know, though, is that it was acquired by the Cora Diamond Corporation in New York, and that cutting it was both a high‑stakes gamble and an extreme technical challenge.

The worry was that heat from the polishing process would destroy the green component - the "defect" from radiation that gave the stone its exceptional value - and turn brown.

Mazhar Saylam, the Turkish-American master diamond cutter and color‑diamond specialist, took charge of the process. To protect the stone, it had to be continuously cooled, which made the task agonizingly slow. But ultimately successful.

Heat wasn't the only concern. The vivid blue‑green hue is concentrated near the surface and outer layers of the stone, so cutting too deeply risked exposing a more colorless core and drastically reducing the gem's saturation and value.

Because the color is not uniform through the entire stone, the cutter had to work without knowing exactly how deep the saturated blue‑green band ran.

Each facet‑planing decision was a calculated risk as to where the color would remain strongest.

Saylam also had to balance weight retention against the need to remove flaws and shape the stone, under intense pressure not to "over‑cut."

The finished product, a fancy trillion (triangular brilliant) gem, with an "arresting electric blue-green hue", was first sold at Christie's Geneva in May 2014 for CHF 7.7 million (equivalent at the time to USD 8.65 million). It returns there on 13 May.

Have a fabulous weekend.