The Blue Moon of Josephine is a 12.03-carat cushion-cut diamond graded by the Gemological Institute of America as Fancy Vivid Blue, Internally Flawless — a confluence of colour, clarity, and size that places it among the most significant blue diamonds ever to appear at public auction. Sold at Sotheby's Geneva on 11 November 2015, the stone achieved a final price of approximately US $48.4 million, establishing a world auction record for price per carat for any diamond at the time: roughly $4.02 million per carat. The purchaser, Hong Kong property developer Joseph Lau Luen-hung, renamed the stone the Blue Moon of Josephine in honour of his daughter Josephine, a gesture that has since become part of the stone's permanent identity in the trade literature.

Discovery and Early History

The rough diamond from which the Blue Moon of Josephine was fashioned was recovered in January 2014 from the Cullinan mine in Gauteng, South Africa — the same storied kimberlite pipe that yielded the two largest polished diamonds in the British Crown Jewels, the Cullinan I and Cullinan II. The Cullinan mine, operated by Petra Diamonds, has historically been one of the world's most prolific sources of large, high-quality Type IIb diamonds, the rare boron-bearing variety responsible for the blue colour seen in stones such as the Hope Diamond. The rough crystal weighed 29.6 carats and was described by Petra at the time of its recovery as an exceptional gem-quality blue diamond. It was subsequently sold to Cora International, a New York-based diamond cutting firm, which undertook the fashioning of the stone.

Fashioning and Gemological Characteristics

Transforming a 29.6-carat rough into a 12.03-carat polished stone represents a yield of approximately 40 per cent — not unusual for a diamond cutter working to maximise colour saturation and clarity in a fancy-colour stone, where the orientation of the crystal relative to the cut can profoundly affect the depth and evenness of hue. The resulting cushion-cut form concentrates the blue body colour effectively, and the Internally Flawless clarity grade confirms the absence of any inclusions visible under ten-times magnification, with only minor surface blemishes that could theoretically be removed by further polishing.

The GIA colour grade of Fancy Vivid Blue is the highest saturation designation the Institute assigns to blue diamonds. In GIA's grading system, blue diamonds may be described as Faint, Very Light, Light, Fancy Light, Fancy, Fancy Intense, Fancy Vivid, Fancy Deep, or Fancy Dark; Fancy Vivid represents the pinnacle of saturation combined with a pure, even hue. Blue diamonds of any size in the Fancy Vivid category are extraordinarily rare; at 12.03 carats, the Blue Moon of Josephine is among the largest polished examples of this grade known to exist.

Type IIb diamonds, the structural category to which virtually all natural blue diamonds belong, contain trace quantities of boron substituted within the crystal lattice in place of carbon. Boron absorbs light in the red and infrared portions of the visible spectrum, producing the characteristic blue body colour. Type IIb stones are also notable for being semiconductors, a property that distinguishes them from the electrically insulating Type Ia and Type IIa diamonds that constitute the vast majority of gem diamonds. The GIA report for the Blue Moon of Josephine confirmed its Type IIb classification.

The 2015 Sotheby's Sale

The stone was offered at Sotheby's Magnificent Jewels and Noble Jewels sale in Geneva on 11 November 2015 under the pre-sale name "The Blue Moon," with a pre-sale estimate that was withheld — a common practice for exceptional lots where the auction house anticipates competitive bidding beyond any published figure. The hammer fell at a price that, with buyer's premium, totalled approximately CHF 48.47 million (US $48.4 million at prevailing exchange rates), surpassing the previous per-carat record for a diamond at auction, which had been held by the Pink Star diamond sold at Sotheby's Hong Kong in 2013 (subsequently rescinded due to non-payment, a complication that briefly returned the Pink Star to the market).

Joseph Lau's acquisition of the Blue Moon of Josephine was part of a broader pattern of significant coloured-diamond purchases he made during this period. On the same evening at Sotheby's Geneva, he also acquired a 16.08-carat Fancy Vivid Pink diamond — subsequently renamed Sweet Josephine — for approximately $28.5 million, also in honour of his daughter. The two purchases together represented one of the most remarkable single-evening expenditures on coloured diamonds in auction history.

Significance in the Coloured Diamond Market

The Blue Moon of Josephine's auction result crystallised several market dynamics that specialists had been observing over the preceding decade. Fancy Vivid Blue diamonds at significant carat weights had been appreciating at rates that outpaced most other asset classes in the luxury sector, driven by the near-total absence of new supply at the top of the quality spectrum and by sustained demand from collectors in Asia, the Middle East, and the United States. The Cullinan mine remains one of the very few reliable sources of large Type IIb blue diamonds; other historically important sources, including the Golconda mines of India that yielded the Hope Diamond, are long exhausted.

The price per carat achieved — approximately $4.02 million — set a benchmark that the trade has used since as a reference point for valuing exceptional blue diamonds. It underscored the exponential, rather than linear, relationship between carat weight and value in the finest coloured diamonds: a 12-carat Fancy Vivid Blue is not merely twelve times as valuable as a one-carat stone of equivalent quality, but potentially many multiples of that, because stones of such size and grade are encountered perhaps once in a generation.

Comparison with Other Famous Blue Diamonds

The Blue Moon of Josephine occupies a distinct position among the world's celebrated blue diamonds. The Hope Diamond, at 45.52 carats, is the largest known Fancy Deep Grayish Blue diamond and resides permanently in the Smithsonian Institution; it has not been available for sale in the modern market. The Wittelsbach-Graff Diamond, a 31.06-carat Fancy Deep Grayish Blue stone, sold at Christie's London in 2008 for £16.4 million before being controversially recut by Laurence Graff to 31.06 carats and regraded. The Blue Moon of Josephine's distinction lies in the combination of the Fancy Vivid grade — the purest and most saturated blue designation — with a carat weight exceeding twelve carats and an Internally Flawless clarity, a trio of attributes that no other publicly documented blue diamond is known to share simultaneously.

Current Status

Following its purchase by Joseph Lau, the Blue Moon of Josephine entered private ownership and has not reappeared at public auction. Like many stones of its calibre, it is presumed to be held as a private collection piece. Its GIA grading report remains the definitive gemological record of its characteristics, and the stone continues to be cited in auction-house literature, gemological publications, and market analyses as a defining transaction in the modern coloured-diamond market.

Further Reading

• GIA — Blue Moon Diamond coverage
• Gems & Gemology — Blue Moon Diamond, Fall 2015