On 25 April 1905, workmen at the Premier Mine (later renamed the Cullinan Mine) near Pretoria, Transvaal Colony, discovered a rough white diamond weighing 3,106 carats (621.2 grams). The stone, later named the Cullinan Diamond after the mine’s owner Thomas Cullinan, remains the largest gem-quality rough diamond ever found. Its scale and quality set it apart from larger non-gem diamonds known from other deposits. The discovery occurred within a kimberlite pipe that had been the focus of prospecting since the late 19th century. The Premier Mine had been producing diamonds since its opening in 1902, and the geology of the site—diamond-bearing kimberlite intrusions—made it one of the most significant diamond sources of the era. The sheer size of the Cullinan made immediate transportation and security a major concern for the mine’s operators and for the government authorities of the Transvaal. After its recovery, the Cullinan was inspected and described by gemologists who noted its exceptional clarity and color. In 1907 the stone was purchased by the Transvaal government and presented to King Edward VII as a gesture of loyalty and to strengthen ties between the colony and the British Crown. The gem was subsequently sent to Asscher Brothers of Amsterdam, renowned diamond cutters, to be cleaved and polished. Cutting such a massive and valuable rough posed unprecedented technical and ethical challenges. The Asschers worked with drawings and careful planning to maximize both carat weight and the number of high-quality finished stones. In 1908 the largest resulting gem, the Cullinan I (also called the Great Star of Africa), weighed 530.2 carats once cut and was mounted in the Sovereign’s Sceptre with Cross. Other major stones produced from the rough include the Cullinan II (317.4 carats), set in the Imperial State Crown, along with several smaller brilliant and pear-shaped stones that entered royal and private collections. The Cullinan’s recovery and subsequent cutting had several wider impacts. Technically, it advanced precision cutting techniques and underscored the importance of professional gemological assessment for exceptional finds. Economically and politically, its presentation to the British monarch became a symbol of the connections between the Transvaal and the British Empire during a period of shifting colonial relations following the South African (Boer) War. The stones produced from the Cullinan remain among the most famous and publicly visible jewels in the British Crown Jewels. While the Cullinan Diamond is the largest gem-quality rough diamond on record, there are larger known diamonds of industrial quality or lower gem grade reported elsewhere and in other eras. Accounts of diamond discoveries in the 19th and early 20th centuries sometimes vary in reported weights or descriptions; where discrepancies exist, contemporary gemological records and museum catalogues provide the most reliable figures. The Cullinan’s documented weight of 3,106 carats for the rough stone and the documented weights of its major polished gems are supported by archival records from the Premier Mine, the Transvaal government, and the Asscher firm’s records. Today, the legacy of the 1905 find endures in both public display and scholarly attention. The largest polished stones remain part of the British Crown Jewels and are periodically exhibited, while the story of the Cullinan continues to be cited in discussions of diamond geology, colonial-era mining history, and the technological advances in gem cutting that early 20th-century demands helped to spur.