The world's largest rough diamond was so valuable that authorities used a decoy ship.

Cultures · Published 10 October 2026 · Updated 25 October 2026 Swipe more like this →

In short: Discovered in 1905, the 3,106-carat Cullinan Diamond was too precious to risk on a single voyage. While detectives guarded a fake stone on a steamship to lure potential thieves, the real gem traveled to London in a plain box via registered post. Its journey had only just begun.

Found 5.5 metres below the surface at South Africa's Premier No.2 mine on 26 January 1905, the Cullinan Diamond remains the largest gem-quality rough diamond ever unearthed. Weighing 3,106 carats, or about 621 grams, it was named after the mine's owner, Thomas Cullinan. Geologists estimate the stone formed in the Earth's mantle transition zone, between 410 and 660 kilometres deep, before reaching the surface 1.18 billion years ago. Its size was staggering, measuring roughly 10 centimetres long and containing a small air pocket that created rainbow-like Newton's rings.

After two years of failing to find a buyer, the Transvaal Colony government purchased the diamond for £150,000 in 1907—a sum equivalent to £21 million today—and presented it to King Edward VII. The task of cutting the massive stone fell to Joseph Asscher & Co. in Amsterdam. The process was fraught with tension; splitting the diamond required weeks of planning and a precise incision. When Joseph Asscher finally cleaved the stone in 1908, it was a technical triumph, though legends of him fainting from the stress are likely apocryphal.

The cutting process yielded nine major stones and 96 minor brilliants. The two largest, Cullinan I and II, are now part of the British Crown Jewels, set into the Sovereign's Sceptre with Cross and the Imperial State Crown, respectively. Other pieces, affectionately dubbed 'Granny's Chips' by the royal family, were frequently worn by Queen Elizabeth II. While the Cullinan I, at 530.4 carats, remains the world's largest colourless cut diamond, it was eventually surpassed in total weight by the Golden Jubilee Diamond in 1992 and the Black Falcon in 2025.

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