The Allnatt Diamond is one of the most celebrated fancy-colour diamonds in existence: a cushion-cut stone of 101.29 carats graded by the Gemological Institute of America (GIA) as Fancy Vivid Yellow with VS2 clarity. Those two designations together — a weight exceeding one hundred carats and the highest possible saturation grade in GIA's fancy-colour nomenclature — place the Allnatt in a category occupied by only a handful of diamonds known to history. Its name derives from Major Alfred Ernest Allnatt, the British soldier, businessman, and philanthropist who owned the stone during the mid-twentieth century and commissioned Cartier to mount it in one of the firm's characteristically refined floral brooches. The diamond has since passed through the international auction market and remains, by all credible accounts, in private hands.

Physical and Optical Character

Yellow colour in diamond arises from the presence of nitrogen atoms substituted into the crystal lattice in aggregated form — specifically the configuration known as the C-centre, or isolated substitutional nitrogen, which absorbs light in the blue and violet regions of the visible spectrum and transmits the complementary yellow. Diamonds of this type are classified by gemologists as Type Ib when nitrogen is dispersed as single atoms, though many intensely yellow stones contain a mixture of nitrogen configurations. The precise structural chemistry of the Allnatt has not been publicly disclosed in the scientific literature, but the depth and evenness of its colour are consistent with an unusually high nitrogen concentration distributed in a manner that produces saturated, face-up yellow without the brownish or greenish modifiers that commonly dilute colour in large stones.

GIA's fancy-colour grading scale for yellow diamonds ascends through Fancy Light, Fancy, Fancy Intense, Fancy Deep, and Fancy Vivid. The Fancy Vivid designation is reserved for stones exhibiting the highest combination of hue, tone, and saturation, and it is statistically uncommon even among yellow diamonds — which are themselves the most frequently encountered fancy colour. To achieve Fancy Vivid at a weight of 101.29 carats, with VS2 clarity, is a convergence of natural circumstances of extraordinary rarity. VS2 clarity indicates very small inclusions that are difficult to locate under 10× magnification and have no meaningful effect on the stone's transparency or brilliance.

The cushion cut — a shape with a rectangular or square outline softened by rounded corners and a facet arrangement descended from the old mine cuts of the eighteenth and nineteenth centuries — suits large, deeply coloured diamonds particularly well. The relatively broad facet table and generous crown angles of a cushion cut allow colour to pool and intensify in the face-up position, a phenomenon sometimes described in the trade as colour return. For a yellow diamond of this saturation, the cushion silhouette maximises the visual impact of the hue while preserving the maximum possible weight from the rough crystal.

Geological Origin and Probable Provenance

The Allnatt Diamond's mine of origin has never been definitively established in the public record. The most plausible candidate, advanced on the basis of the stone's colour character and the historical period of its emergence, is the Premier Mine (now the Cullinan Mine) in Gauteng, South Africa. The Premier Mine has yielded a disproportionate share of the world's largest and most historically significant diamonds, including the Cullinan Diamond (3,106 carats in the rough), the Taylor-Burton Diamond, and several other major fancy-colour stones. The mine's kimberlite pipe is geologically unusual in producing diamonds with both exceptional size and a tendency toward strong yellow and blue colours. However, in the absence of documented chain of custody from the mine to Major Allnatt, the Premier Mine attribution must be treated as probable rather than confirmed.

Diamonds of this scale originate in the lithospheric mantle, typically at depths of 150 kilometres or more, where carbon crystallises under pressures exceeding 45 kilobars and temperatures above 900 degrees Celsius. The incorporation of nitrogen during crystal growth, and the subsequent thermal history that determines how nitrogen aggregates within the lattice, ultimately governs the colour that will be expressed when the diamond reaches the surface via kimberlite eruption. A stone of 101 carats in the cut represents a rough crystal of perhaps 200 to 250 carats or more, depending on the yield of the cutting process — itself a testament to the size of the original octahedral or macle crystal from which it was fashioned.

Major Alfred Ernest Allnatt

Alfred Ernest Allnatt (1896–1969) was a British Army officer, property developer, and philanthropist whose career bridged military service in both World Wars and a subsequent fortune built largely through post-war London property. He was known as a collector of art and objects of exceptional quality, and his acquisition of the yellow diamond — most likely in the early to mid-1950s — was consistent with a pattern of connoisseurship that extended to paintings, horses, and charitable endowments. Allnatt donated generously to ecclesiastical causes, most notably funding the restoration of the high altar tapestry at Coventry Cathedral.

His commission to Cartier for the mounting of the diamond reflects the taste of the period and the social register of the owner. Cartier's Paris and London workshops were the natural destination for a collector of Allnatt's standing seeking a setting worthy of a stone of this importance. The resulting brooch, designed in the form of a flower with the yellow diamond as its central element and smaller white diamonds forming petals and stem work, exemplifies the mid-century Cartier aesthetic: technically accomplished, restrained in its deployment of secondary stones so as not to compete with the principal gem, and conceived as a wearable jewel rather than a static display piece.

The Cartier Setting

Cartier's involvement with the Allnatt Diamond is well documented in the context of the stone's auction history and has been referenced in major sale catalogues. The flower brooch format was a recurring motif in Cartier's mid-twentieth-century production, appearing across a range of scales and materials. For the Allnatt commission, the challenge was to create a mount that could accommodate a stone of over one hundred carats — a weight that imposes significant structural demands on any brooch fitting — while maintaining the visual lightness and botanical naturalism characteristic of the firm's floral work. The use of white diamonds as secondary stones serves a dual purpose: it provides the necessary contrast to make the yellow of the central stone appear even more saturated by simultaneous contrast, and it anchors the piece within the tradition of colourless diamond jewellery that formed the core of Cartier's commercial identity.

Auction History and Market Significance

The Allnatt Diamond was offered at Christie's Geneva in May 1996, where it achieved a sale price reported at the time as approximately 3.04 million Swiss francs — a figure that, even accounting for subsequent currency movements and the dramatic appreciation of the fancy-colour diamond market in the intervening decades, represented a significant transaction for a stone of this character. The 1996 sale established a documented public price point for the diamond and confirmed its status as a museum-quality object in the international auction market.

Since 1996, the fancy-colour diamond market has undergone a transformation in both depth and pricing. Stones graded Fancy Vivid Yellow at weights above fifty carats now routinely achieve prices per carat that would have been difficult to anticipate in the mid-1990s, driven by increased participation from Asian collectors, growing awareness of GIA's fancy-colour grading system as a reliable benchmark, and the broader financialisation of trophy gemstones as a category of portable, stateless wealth. The Allnatt, at 101.29 carats Fancy Vivid Yellow VS2, occupies a position near the apex of this market. Estimates of its current value, where they appear in credible trade and auction-house commentary, have consistently exceeded ten million United States dollars, with some assessments placing it considerably higher depending on market conditions at the time of any hypothetical future sale.

The stone's combination of attributes — century-scale weight, the highest GIA colour grade, VS2 clarity, distinguished mid-century provenance, and a documented Cartier mounting — constitutes what auction specialists sometimes describe as a confluence of factors: each attribute individually would be noteworthy; together, they produce a stone for which direct comparables are essentially non-existent.

The Allnatt in the Context of Major Yellow Diamonds

Among the world's most significant yellow diamonds, the Allnatt occupies a position defined by its weight and colour grade. For context, the Tiffany Yellow Diamond, perhaps the most famous yellow diamond in the world, weighs 128.54 carats in its current cushion cut and has been described as Fancy Yellow (the precise GIA grade has been reported variously in the literature, and the stone predates GIA's modern fancy-colour grading system in its original cutting). The Incomparable Diamond, a brownish-yellow stone of 407.48 carats, exceeds the Allnatt substantially in weight but carries a colour modifier that places it in a different aesthetic category. The Sun-Drop Diamond, a pear-shaped Fancy Vivid Yellow of 110.03 carats sold at Sotheby's Geneva in 2011 for approximately 12.4 million Swiss francs, provides perhaps the closest modern auction comparable in terms of colour grade and approximate weight, though the two stones differ in cut, clarity, and provenance.

What distinguishes the Allnatt within this peer group is the combination of a round-cornered cushion cut — historically the most prestigious shape for large diamonds — with Fancy Vivid saturation at a weight that crosses the psychologically and commercially significant threshold of one hundred carats. Diamonds above one hundred carats are sufficiently rare that they are individually named and tracked; Fancy Vivid yellow diamonds above one hundred carats are, by any reasonable count, among the rarest natural objects in private ownership.

Treatment Status

No credible source has suggested that the Allnatt Diamond has been subjected to colour enhancement. Yellow colour produced by nitrogen aggregation is entirely natural and stable, and there is no documented treatment capable of converting a colourless or near-colourless diamond into a Fancy Vivid Yellow of this depth and evenness without producing detectable spectroscopic signatures. GIA's grading report for the Allnatt, issued in conjunction with the 1996 Christie's sale, confirmed the colour as natural — a designation that, for a stone of this importance, is both commercially essential and scientifically credible given the nature of nitrogen-induced yellow colour in diamond.

Current Status

The Allnatt Diamond is understood to remain in private ownership following its 1996 Christie's sale. It does not appear in the permanent collection of any public museum or institution, and it has not been offered at public auction in the period since 1996 for which reliable records are available. Its periodic appearance in discussions of the world's most important fancy-colour diamonds — in auction-house reference materials, gemmological literature, and specialist journalism — reflects the enduring significance of its specifications and the relative scarcity of stones that can be discussed in the same breath.

Further Reading

• GIA: Fancy Colour Diamonds — Research and Grading
• Gems & Gemology: Fancy-Colour Diamonds (GIA)
• GIA: The 4Cs of Diamond Quality