The Blue Heart diamond is a 30.62-carat fancy deep greyish-blue diamond cut in the form of a heart, and one of the most distinctive large blue diamonds held in any public institution. It resides today in the National Gem Collection of the Smithsonian Institution's National Museum of Natural History in Washington, D.C., where it has been on permanent display since 1964. Its provenance traces a characteristically glamorous arc through early-twentieth-century Paris, the salons of Argentine high society, and the celebrated collecting life of American heiress Marjorie Merriweather Post — a journey that mirrors, in miniature, the broader story of how the world's great coloured diamonds migrated from private hands into museum custody during the twentieth century.

Physical and Optical Character

Blue diamonds owe their colour to the presence of boron atoms substituted into the diamond crystal lattice, a structural impurity that absorbs red, orange, and yellow wavelengths of light and transmits the blue end of the visible spectrum. The Blue Heart's colour grade — fancy deep greyish-blue in the terminology of the Gemological Institute of America's coloured-diamond grading system — indicates a saturated, somewhat darkened blue with a secondary grey modifier, a combination that gives the stone a steely, velvety depth rather than the vivid sky-blue of, say, the Hope Diamond. Both the Hope and the Blue Heart are classified as Type IIb diamonds, the rare subtype characterised by boron impurities and the near-absence of nitrogen, which also confers weak phosphorescence and mild electrical semi-conductivity.

At 30.62 carats, the Blue Heart is a stone of substantial size. Large blue diamonds of any shade are among the rarest objects in the natural world; the combination of that size with a heart-shaped outline places it in a still smaller category. The heart shape — technically a modified brilliant cut with a cleft at the top and a pointed culet — demands exceptional skill from the cutter, who must balance symmetry across the two lobes while preserving as much rough weight as possible and maintaining the optical performance of the faceting. The stone's precise cutting history is not fully documented in the public record, but it was almost certainly fashioned in the early years of the twentieth century, when Parisian lapidaries were at the forefront of fancy-shape diamond cutting.

Acquisition by Cartier and Early Provenance

The earliest well-documented chapter of the Blue Heart's modern history begins in 1909, when the house of Cartier acquired the stone in Paris. At that moment, Cartier — under the direction of Louis Cartier — was at the height of its influence as both jeweller and dealer in important gemstones, regularly buying and selling significant diamonds as commercial inventory rather than simply mounting stones to commission. The firm's ledgers and correspondence from this period record transactions involving numerous large coloured diamonds, and the Blue Heart appears to have passed through the house as part of this trade rather than being set into a major Cartier jewel that survives as a documented object.

The stone is also known by the alias Eugénie Blue, a name that connects it — at least nominally — to Empress Eugénie de Montijo, consort of Napoleon III and one of the most celebrated jewellery collectors of the nineteenth century. Eugénie's collection included several important blue diamonds, and her name has become loosely attached to a number of stones whose precise histories overlap with the Second Empire period. However, no documentary evidence firmly establishes that this particular diamond was ever owned by or made for the Empress, and the association must be treated as unverified tradition rather than established provenance. The alias persists in the trade and in museum literature as a secondary name, but the Smithsonian catalogues the stone under its primary designation, the Blue Heart.

Argentine Ownership: Unzué de Alvear

After its passage through Cartier, the Blue Heart entered the possession of a member of the Argentine aristocracy, identified in Smithsonian records as Mrs. Unzué de Alvear. The Unzué and Alvear families were among the most prominent landowning dynasties of late-nineteenth and early-twentieth-century Argentina, their fortunes built on the cattle estancias of the Pampas and consolidated through strategic marriages and political connections. Members of these families were regular visitors to Paris and London, and the acquisition of important jewels from leading European houses was entirely consistent with the social practices of this milieu.

The precise date of Mrs. Unzué de Alvear's acquisition and the circumstances of the purchase are not fully documented in sources available to the public record. What is established is that the stone remained in her possession for a substantial period and that she chose to bequeath it not to a family member but to a close personal friend: the American socialite and philanthropist Marjorie Merriweather Post.

Marjorie Merriweather Post and the Path to the Smithsonian

Marjorie Merriweather Post (1887–1973) was one of the most consequential private collectors of gemstones and decorative arts in twentieth-century America. Heir to the Post Cereals fortune (later General Foods), she assembled collections of Imperial Russian objects, French decorative arts, and jewellery of exceptional quality, housed variously at her Palm Beach estate Mar-a-Lago, her Adirondacks camp Topridge, and her Washington residence Hillwood — the last of which she converted into a museum that continues to operate today as Hillwood Estate, Museum and Gardens.

Post's jewellery collection was distinguished by its emphasis on historical importance and provenance as much as sheer monetary value. Her acquisition of the Blue Heart through bequest from Mrs. Unzué de Alvear placed her in possession of one of the world's significant blue diamonds at a moment when she was already contemplating the disposition of her most important objects. In 1964, Post donated the Blue Heart to the Smithsonian Institution's National Gem Collection — the same repository that had received the Hope Diamond from Harry Winston in 1958. The two donations, separated by only six years, gave the Smithsonian an unparalleled concentration of large blue diamonds in public hands.

Post's decision to give the stone to the Smithsonian rather than sell it or retain it within her estate reflected both her philanthropic philosophy and a broader mid-century conviction among major American collectors that objects of national significance belonged in public institutions. The Blue Heart joined the National Gem Collection as a gift of genuine cultural weight, and it has remained on display there continuously since its donation.

The Blue Heart at the Smithsonian

Within the National Museum of Natural History's Janet Annenberg Hooker Hall of Geology, Gems, and Minerals, the Blue Heart occupies a position among the collection's most celebrated coloured diamonds. It is displayed in proximity to the Hope Diamond — the 45.52-carat fancy deep greyish-blue cushion-cut stone that serves as the collection's centrepiece — allowing visitors to make a direct visual comparison between two of the world's most important blue diamonds of similar colour character.

The juxtaposition is instructive. Both stones share the fancy deep greyish-blue colour description and both are Type IIb, yet they differ markedly in cut, outline, and visual personality. The Hope's cushion shape and antique faceting pattern give it a brooding, almost architectural presence; the Blue Heart's symmetrical lobes and brilliant-style faceting produce a more openly luminous effect, the heart outline lending it an immediate visual legibility that the Hope's more complex silhouette does not share. Together they illustrate the range of appearance possible within a single colour grade and diamond type.

The Smithsonian's curatorial records describe the stone as set in a platinum and diamond mounting, though the stone has been remounted at various points in its history and the current setting is not the original Cartier context, if indeed Cartier ever mounted it at all. The heart shape is displayed point-downward in the conventional orientation for heart-shaped stones in pendant or brooch settings.

Scientific Significance

As a Type IIb diamond, the Blue Heart is of genuine scientific interest beyond its aesthetic and historical value. Type IIb stones are extraordinarily rare — they constitute well under one per cent of all gem diamonds — and large examples suitable for detailed spectroscopic study are rarer still. The boron content responsible for the blue colour also makes these diamonds weak semiconductors, a property unique among gem materials and one that has attracted sustained interest from solid-state physicists and materials scientists.

The Smithsonian's collection of Type IIb diamonds, which includes the Hope, the Blue Heart, and several other significant stones, has supported published research into the optical and electronic properties of natural boron-doped diamond. Studies using photoluminescence spectroscopy, infrared absorption, and phosphorescence measurement have contributed to the broader scientific understanding of how boron interacts with the diamond lattice and how the resulting colour and electronic properties vary with boron concentration and distribution. The Blue Heart's deep greyish-blue colour, slightly different in character from the Hope's, reflects subtle differences in boron content and possibly in the distribution of other trace elements, making it a useful comparative specimen.

The Name "Eugénie Blue" and the Question of Imperial Provenance

The alias Eugénie Blue deserves careful treatment. Empress Eugénie (1826–1920) was a passionate collector of important jewels, and her collection — dispersed in part after the fall of the Second Empire in 1870 and again after her death — included several notable blue diamonds. The French crown jewels, with which she had close association, contained the historic Sancy Diamond and other significant stones, and her personal collection was extensive. It is not implausible that a blue diamond of this quality could have passed through her hands or been acquired for her.

However, the diamond trade of the nineteenth century was not characterised by the rigorous provenance documentation that modern auction houses and laboratories now require, and many stones acquired romantic imperial associations through dealers' marketing rather than through verifiable chain of ownership. The name Eugénie Blue appears to have attached to this stone at some point in the late nineteenth or early twentieth century, possibly during or after its passage through the Paris market, but no auction catalogue, inventory, or correspondence has been published that places the stone definitively in Eugénie's collection. The Smithsonian does not endorse the imperial attribution in its primary documentation, and gemmological prudence requires treating it as unconfirmed.

This is not unusual in the history of famous diamonds. The Hope Diamond itself carries a chain of associations — including a supposed curse and a claimed connection to the French Blue stolen during the Revolution — some of which are well-documented and some of which are embellishment. The Blue Heart's Eugénie association belongs to the same tradition of romantic attribution that surrounds many great stones, and it adds to the stone's cultural texture without constituting reliable historical fact.

Place in the Canon of Famous Blue Diamonds

The world's canon of historically significant large blue diamonds is small. Beyond the Hope and the Blue Heart, it includes the Wittelsbach-Graff (35.56 carats, fancy deep greyish-blue, recut controversially in 2008), the Blue Moon of Josephine (12.03 carats, fancy vivid blue, sold at Sotheby's Geneva in 2015 for a then-record per-carat price for any gemstone), and the Oppenheimer Blue (14.62 carats, fancy vivid blue, sold at Christie's Geneva in 2016). Each of these stones occupies a distinct position in terms of colour character, cut, size, and provenance narrative.

The Blue Heart's distinction within this group rests on several factors: its heart-shaped outline, which is unique among the major named blue diamonds; its early-twentieth-century Cartier provenance; its passage through South American aristocratic ownership, a relatively unusual chapter in the biography of a great diamond; and its long tenure in one of the world's premier public gem collections. At 30.62 carats, it is large enough to command serious attention in any context, and its colour — deep, slightly greyed, velvety — is precisely the character that connoisseurs of blue diamonds find most compelling: not the bright, almost electric blue of a vivid-grade stone, but a colour of greater complexity and depth.

Further Reading

• GIA Gems & Gemology — Blue Heart Diamond
• Smithsonian National Museum of Natural History — National Gem Collection
• GIA Research — Type IIb Diamonds and the Hope Diamond