The Cullinan I, known formally as the Great Star of Africa, is the largest polished diamond in the world, weighing 530.20 carats. A pear-shaped brilliant of 74 facets measuring approximately 58.9 × 45.4 × 27.7 mm, it was fashioned in 1908 from the Cullinan rough — itself the largest gem-quality rough diamond ever recovered — and is today mounted in the head of the Sovereign's Sceptre with Cross, one of the most sacred objects of the British Crown Jewels. The stone is a Type IIa diamond, a classification denoting the near-total absence of nitrogen impurity, and it exhibits a colourlessness and transparency of a quality that places it beyond any conventional grading scale. It is held in permanent public display at the Jewel House, Tower of London.

The Cullinan Rough: Discovery and Dimensions

The stone from which Cullinan I was cut was found on 26 January 1905 at the Premier Mine (now the Cullinan Mine) near Pretoria, in what was then the Transvaal Colony of South Africa. The superintendent of the mine, Frederick Wells, noticed a flash of reflected light in the wall of the open pit at a depth of about nine metres; the object he retrieved was a rough diamond of 3,106.75 carats — roughly the size of a man's fist and weighing approximately 621 grams. It was named after Sir Thomas Cullinan, the mining entrepreneur who had opened the Premier Mine in 1902.

The rough displayed a single cleavage face, indicating that it was almost certainly a fragment of a far larger crystal. Despite considerable geological investigation of the Premier Mine over subsequent decades, no matching piece has ever been identified with certainty, though the mine has continued to yield exceptional diamonds, including several large Type IIa stones that have been described as potential siblings to the Cullinan.

The Transvaal government purchased the rough in 1907 for £150,000 and presented it to King Edward VII as a birthday gift, a gesture laden with political significance: the Transvaal had achieved self-governing status only the previous year, and the gift was intended to signal loyalty to the Crown following the trauma of the Anglo-Boer War. The stone was transported to Britain under conditions of considerable secrecy — a decoy parcel was sent by steamship while the actual diamond travelled in a plain box by ordinary post.

Cutting: Joseph Asscher and the Division of the Cullinan

The task of cleaving and polishing the Cullinan rough was entrusted to the Amsterdam firm of I.J. Asscher & Co. (later Joseph Asscher & Co.), then the most respected diamond-cutting house in the world. Joseph Asscher himself studied the stone for several months, mapping its internal grain and the position of inclusions, before making the critical decision about how to divide it. The rough was ultimately cleaved into three primary pieces, from which nine principal stones and approximately 96 smaller brilliants and polished fragments were eventually cut.

The cleaving of the Cullinan has passed into gemmological legend. On 10 February 1908, Asscher placed a specially made steel blade in a prepared groove on the stone's cleavage plane and struck it with a mallet. The blade broke. A second blade was inserted, and on the second blow the stone cleaved perfectly along the intended plane. Accounts differ as to whether Asscher fainted from the relief of the moment or remained composed; the story of his fainting has been widely repeated but is difficult to verify from contemporary sources. What is documented is that the cleaving operation was observed by a small group of witnesses and that the results were precisely as planned.

The nine principal stones were numbered Cullinan I through Cullinan IX in descending order of weight. Cullinan I, at 530.20 carats, and Cullinan II, at 317.40 carats (now set in the Imperial State Crown), are by far the largest. The remaining seven range from 94.40 carats (Cullinan III) down to 4.39 carats (Cullinan IX). The Asscher firm retained the smaller brilliants and polished fragments as partial payment for their work.

Physical and Optical Character

Cullinan I is classified as a Type IIa diamond — a category representing perhaps two per cent of all gem diamonds — in which nitrogen, the most common impurity in diamond, is present at levels below the detection threshold of infrared spectroscopy. Type IIa stones are typically the most optically transparent of all diamonds, transmitting ultraviolet radiation that Type Ia stones absorb, and they frequently display the highest degrees of colourlessness. The Cullinan rough and all nine principal stones cut from it are Type IIa, a fact that underscores the exceptional geological conditions of their formation in the lithospheric mantle beneath the Kaapvaal Craton.

The finished Cullinan I is graded as D colour — the highest grade on the GIA scale — and is considered internally flawless or very nearly so, though formal laboratory grading of stones set in historic regalia presents obvious practical difficulties. Its pear-brilliant form, with 74 facets rather than the 58 of a standard round brilliant, was chosen to maximise the retention of carat weight from the cleaved piece while achieving the most optically effective geometry. The stone's dimensions — approximately 58.9 mm along its longest axis — make it visually extraordinary even in photographs; in person, the effect has been described by curators and conservators as unlike any other polished diamond.

Setting: The Sovereign's Sceptre with Cross

Cullinan I was set into the Sovereign's Sceptre with Cross in 1910, replacing a large spinel that had previously occupied the principal setting. The Sceptre itself dates to the Restoration of the monarchy in 1661, when the regalia destroyed under the Commonwealth was remade for the coronation of Charles II. It is one of the most symbolically charged objects in the Crown Jewels, representing the sovereign's temporal power, and is placed in the monarch's right hand during the coronation ceremony at Westminster Abbey.

The engineering of the setting was designed to allow Cullinan I to be removed and worn separately as a pendant or brooch — a practical consideration given the stone's value and the desire of the royal family to use it as personal jewellery. Queen Mary wore it on several occasions, and Queen Elizabeth II was photographed wearing it as a brooch. The detachable mount is a feature that distinguishes the Cullinan I setting from most other Crown Jewels settings, which are fixed.

The Sceptre, with Cullinan I in place, was used at the coronation of George V in 1911, George VI in 1937, and Elizabeth II in 1953. It was used again at the coronation of Charles III on 6 May 2023, making Cullinan I an active participant in the living ceremonial tradition of the British monarchy rather than merely a museum object.

The Question of Ownership and Repatriation

The Cullinan diamond has periodically been the subject of calls for repatriation to South Africa, where it was mined. The political context of its acquisition — a gift from a newly self-governing colony to an imperial monarch — has drawn renewed scrutiny in the broader global conversation about the provenance of objects held in European collections. The South African government and various civil society organisations have at different times raised the question of whether the stone should be returned.

The British royal family and the British government have consistently maintained that the Cullinan was a freely given diplomatic gift, not a looted object, and that its status within the Crown Jewels is therefore distinct from that of objects taken under colonial duress. The debate reflects genuinely complex questions about the nature of colonial-era gifts, the degree of agency possessed by the Transvaal government at the time of the presentation, and the appropriate framework for evaluating such transactions from a contemporary ethical standpoint. As of the time of writing, no formal repatriation process has been initiated.

The Premier Mine and the Cullinan Legacy

The Premier Mine, renamed the Cullinan Mine in 2003, remains one of the most important diamond-producing operations in the world and the primary source of large Type IIa diamonds. It has yielded a remarkable series of exceptional stones in the century since the original Cullinan discovery, including the 507.55-carat rough that produced the Golden Jubilee diamond (545.67 carats polished, the largest faceted diamond in the world by weight, though not by the prestige accorded to Cullinan I), and the 122.52-carat Cullinan Heritage rough sold in 2010. The mine's geological character — a kimberlite pipe of unusual depth and extent, tapping a lithospheric root of exceptional antiquity — appears to favour the formation of large, nitrogen-poor crystals.

The Asscher firm, which cut the original Cullinan, continues to operate as the Royal Asscher Diamond Company, and the cutting of the Cullinan rough remains the defining episode in its history. The firm has maintained meticulous records of the operation and has been a primary source for historical accounts of the cleaving and polishing process.

Cullinan I in the Context of World Diamonds

To appreciate the scale of Cullinan I, it is useful to place it alongside the other great polished diamonds of history. The Koh-i-Noor, perhaps the most famous diamond in the world by name recognition, weighs 105.60 carats in its current oval brilliant form. The Hope Diamond, celebrated for its deep blue colour, weighs 45.52 carats. The Centenary Diamond, a modern Type IIa stone cut in 1991 and widely regarded as one of the finest diamonds ever polished, weighs 273.85 carats. Against these benchmarks, Cullinan I's 530.20 carats represents a mass roughly double that of the next largest polished diamond of comparable quality — Cullinan II — and nearly five times the weight of the Koh-i-Noor.

The stone's status as the world's largest polished diamond is not merely a matter of historical record; it reflects a physical reality about the rarity of large, gem-quality rough diamonds that makes it extremely unlikely to be surpassed. Rough diamonds of sufficient size and quality to yield a polished stone exceeding 530 carats are, on current evidence, vanishingly rare. The Cullinan rough itself was so anomalous that it remained unmatched for more than a century, and no subsequent discovery has approached its dimensions in combination with gem quality.

Display and Access

The Sovereign's Sceptre with Cross, bearing Cullinan I, is displayed in the Jewel House at the Tower of London alongside the other principal items of the Crown Jewels. The Jewel House receives approximately 2.5 million visitors per year, making the Crown Jewels collection one of the most visited museum displays in the world. The viewing conditions — a slow-moving walkway past illuminated cases — are designed to allow close observation of the stones while maintaining security and managing visitor flow. Cullinan I is sufficiently large that its form and faceting are clearly legible to the naked eye at normal viewing distances, an experience that distinguishes it from most famous diamonds, which are small enough to require magnification for full appreciation.

The Tower of London is administered by Historic Royal Palaces, an independent charity, and the Crown Jewels are the property of the Crown rather than the state, held in trust for future sovereigns. This legal status has implications for questions of sale, loan, or repatriation, as the items cannot be disposed of by the government acting alone.

Further Reading

• GIA Gems & Gemology: The Cullinan Diamonds (Fall 2010)
• GIA Gem Encyclopedia: Diamond