GENEVA - Once in a blue moon, the earth sends forth a coloured diamond. Intense and intriguing, the stone may be yellow, pink, orange or violet, yet its fiery brilliance and exquisite colour – the most powerful emotional triggers in a gem – always come together with dramatic effect. Among these coveted stones (there is one for every 10,000 gem-quality white diamonds found), the blue diamond is considered the rarest, most captivating and most enigmatic treasure.

A fabulous fluke of nature, a blue diamond is produced by the random presence of the atomic lattice-bound trace element boron within the stone’s carbon structure during its formation deep in the earth’s core. The earliest known blue diamond came from the legendary Kollur mine in the Golconda, a region of India that produced such famous examples as the Idol’s Eye (15th century) and the Hope Diamond (17th century), perhaps the most fabled gemstone on the planet. Later, in the early 18th century, blue diamonds were occasionally found in Brazil. But today the Premier Mine at Cullinan, some 40 kilometres east of Pretoria, South Africa, is the only one known to yield blue diamonds with any regularity. However, discoveries are still sporadic and always astonishing occurrences. Indeed, since the mine was acquired by Jersey-based Petra Diamonds in 2008, out of the eighteen million tonnes mined and five million carats recovered, only five world-class blue diamonds have emerged from Cullinan, or less than 0.1 per cent of the mine’s annual yield.

Its softness and noble lines not only celebrate the stone’s other-worldliness, but also enhance its tone and saturation.


All of that changed in January 2014 with the appearance of a 29.62-carat rough blue diamond crystal. Following much press excitement, the diamond was purchased by Cora International, one of the world’s leading firms specialising in cutting large important diamonds. Cora’s experts spent approximately six months deliberating, planning and painstakingly cutting the rough into a magnificent cushion-cut 12.03-carat diamond of perfect proportions. In its detailed monograph, the Gemological Institute of America (GIA) classified the diamond as “fancy vivid blue” and “internally flawless,” hailing it as a “rare wonder.” A pinnacle of perfection with top classifications, the stone was named the Blue Moon and garnered much attention when it went on view at the Natural History Museum of Los Angeles County in August 2014. But when the exhibition closed this past January, the Blue Moon retreated to a vault.

One of the largest known fancy vivid blue diamonds, the Blue Moon is now re-emerging as the dazzling centrepiece of Sotheby’s November sale of Magnificent Jewels and Noble Jewels in Geneva. The last time a vivid blue diamond was polished from the rough was in 2009, when the 7.03-carat Star of Josephine, from the same Cullinan mine as the Blue Moon, fetched $9.5 million at Sotheby’s Geneva, then a record price per carat for any diamond.

The Blue Moon will be collectors’ ultimate object of desire, as well as one of tomorrow’s historic diamonds. For David Bennett, Sotheby’s Worldwide Chairman for Jewellery, the Blue Moon represents “the most mysterious and romantic” of coloured diamonds, its deep blue giving it a hypnotic magnetism. Almost other-worldly, unknowable and absolute, this spectacular specimen is simply awe-inspiring.

Beyond exceptional colour, brilliance and clarity, the Blue Moon, classified as a type IIb diamond, radiates with a captivating inner life. According to the GIA, type IIb diamonds are electrically conductive and known to phosphoresce, usually turning dark blue or green under ultraviolet light. Remarkably, the Blue Moon phosphoresces red, which infuses it with a flaming passion, another extraordinary quality.

Maximising these natural gifts, the Blue Moon also demonstrates diamond cutting’s new levels of excellence and expertise. The world’s elite diamond cutters have the ability to see into the heart of a stone, unveiling its secrets while understanding its intricate structure. Cutting and polishing a coloured diamond of immense value requires exceptional intuition, skill and experience, not to mention diamantine nerves, which the Blue Moon’s realisation embodies to the highest degree. Its exquisite cutting is clearly the result of a true art form, a blend of art and science, of the superrational with the hyper-instinctive. It should be noted that blue diamonds pose very specific challenges to gem cutters. Unlike white diamonds, type IIb blue diamonds are rarely symmetrical in structure; their internal angles are often likened to the knots in a piece of wood, making the cutter’s work all the more perilous. In addition, such stones often reveal uneven colour distribution, known as colour zoning, another parameter for the cutter to consider. One miscalculation, one slip, can mean the colour disappears entirely.

But while a blue diamond may present the greatest challenge for the diamond cutter, technical advances and increased understanding of coloured diamonds have also enabled experts to deepen precious stones’ intensity and illuminate their hues more than ever before. That much is evident in the Blue Moon’s cushion brilliant cut: its softness and noble lines not only celebrate the stone’s other-worldliness, but also enhance its tone and saturation, transforming our perception of the coloured diamond from mineralogical curiosity to refined work of art – impossibly rare, limited only by nature and discovered only once in a blue moon.