The Cullinan diamond is the largest gem-quality rough diamond ever recovered, weighing 3,106.75 carats (approximately 621 grammes) in its uncut state. It was found on 26 January 1905 at the Premier Mine in Cullinan, Gauteng, South Africa — a deposit that had already established itself as exceptional among the world's kimberlite pipes. Named after Sir Thomas Cullinan, the mine's principal owner, the rough was of exceptional transparency and belonged to the rare Type IIa category: chemically pure, essentially free of nitrogen impurities, and capable of transmitting ultraviolet light in a manner that gives such stones their characteristically fine, near-colourless appearance. Its dimensions at discovery were approximately 100 × 65 × 50 millimetres, and one face showed a cleavage plane suggesting it may have been a fragment of a still-larger crystal — a possibility that has never been resolved with certainty.

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Discovery and Acquisition

The stone was discovered by Frederick Wells, the mine's surface manager, who reportedly spotted it protruding from the mine wall during a late-afternoon inspection. Its sheer size initially prompted scepticism: staff at the Premier Mine were said to have suspected a planted hoax before its authenticity was confirmed. The rough was transported to London for examination and subsequently purchased in 1907 by the Transvaal government for £150,000 — a sum that reflected both its extraordinary size and the political symbolism of the transaction. The Transvaal, newly incorporated into the British Empire following the Second Boer War, presented the diamond to King Edward VII as a gesture of loyalty, the gift being formally accepted on the monarch's sixty-sixth birthday, 9 November 1907.

Cleaving and Cutting

The task of dividing and fashioning the rough was entrusted to Joseph Asscher & Co. of Amsterdam, then among the most respected diamond-cutting houses in the world. Joseph Asscher himself studied the stone for several months before committing to a cleaving plan. The first cleavage blow, struck on 10 February 1908, reportedly broke the cleaving blade rather than the diamond; a second blow succeeded. Contemporary accounts — though some details have been embellished in retelling — describe Asscher as having fainted after the successful strike, though the firm's own records are more measured on this point.

The rough was ultimately divided into nine principal gems, designated Cullinan I through IX, along with 96 smaller brilliants and approximately 9.5 carats of unpolished fragments. The cutting work employed a team of three cutters working for eight months. The nine major stones range in weight from Cullinan I at 530.20 carats down to Cullinan IX at 4.39 carats, and include a variety of cuts — pear shapes, marquises, cushions, and an oblong brilliant — each dictated by the geometry of the cleaved pieces rather than by fashion alone.

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The Nine Principal Gems

• Cullinan I (530.20 ct) — a pear-shaped brilliant of 74 facets, also known as the Star of Africa. It is mounted in the Sovereign's Sceptre with Cross and remains the largest colourless cut diamond in the world.
• Cullinan II (317.40 ct) — a cushion-shaped brilliant set in the Imperial State Crown, worn at the State Opening of Parliament. It is the second-largest colourless cut diamond in the world.
• Cullinan III (94.40 ct) — a pear-shaped brilliant, part of the personal jewellery of the British royal family; Queen Mary wore it as a pendant.
• Cullinan IV (63.60 ct) — a cushion brilliant; together with Cullinan III it forms a brooch sometimes called the Granny's Chips brooch, a nickname attributed to Queen Elizabeth II.
• Cullinan V (18.80 ct) — a heart-shaped brilliant, set in a brooch originally made for Queen Mary.
• Cullinan VI (11.50 ct) — a marquise brilliant, given by King Edward VII to Queen Alexandra.
• Cullinan VII (8.80 ct) — a marquise brilliant, suspended from Cullinan VI in a brooch.
• Cullinan VIII (6.80 ct) — an oblong brilliant set in a brooch with Cullinan VI.
• Cullinan IX (4.39 ct) — a pear-shaped brilliant set in a ring made for Queen Mary.

Cullinan I and II are held in the Tower of London as part of the Crown Jewels and are among the most visited gemological objects in the world. The remaining seven stones remain in private royal ownership.

Gemological Character

As a Type IIa diamond, the Cullinan belongs to a class representing fewer than two per cent of all gem diamonds. Type IIa stones contain no measurable nitrogen in their crystal lattice, which accounts for their exceptional colourlessness and, in many cases, their superior transparency across a broad optical spectrum. The Cullinan's colour grade, assessed retrospectively by modern standards, is generally placed in the D–E range. Its clarity, given the size of the rough and the nature of the Premier Mine's production, was remarkable: the principal cut stones are internally clean or very slightly included, a fact that further underscores the extraordinary nature of the original crystal.

The Premier Mine — renamed the Cullinan Mine in 2003 — has continued to yield exceptional Type IIa diamonds, including the 507.55-carat Cullinan Heritage (2009) and several large blue diamonds of note, suggesting that the kimberlite pipe responsible for the original Cullinan rough remains geologically distinctive.

Historical and Cultural Significance

The Cullinan's presentation to Edward VII was not merely a diplomatic gesture; it was a carefully considered act of post-war reconciliation between the Boer-governed Transvaal and the British Crown. The decision to send the rough — rather than a cut stone — to London was itself significant, reflecting a desire to involve British institutions in the stone's transformation. The choice of a Dutch cutting house, however, acknowledged Amsterdam's centuries-long supremacy in diamond fashioning and the practical reality that no British firm of comparable expertise existed at the time.

The diamond has periodically re-entered public discourse in the context of debates over colonial-era acquisitions and repatriation. The South African government has at various points raised the question of the stone's return, though no formal repatriation claim has been lodged through diplomatic channels as of the time of writing. The Cullinan Mine itself is today operated by Petra Diamonds and remains one of the world's most productive sources of large, high-quality rough diamonds.

Further Reading

• GIA Gems & Gemology — Cullinan Diamond History
• GIA Diamond Overview