we know that diamonds did not form from coal because
Diamond A common misconception is that diamonds are formed from highly compressed coal. Coal is formed from buried prehistoric plants, and most diamonds that have been dated are far older than the first land plants. It is possible that diamonds can form from coal in subduction zones, but diamonds formed in this way are rare, and the carbon source is more likely carbonate rocks and organic carbon in sediments, rather than coal.[17][18]
Cullinan Diamond The Cullinan Diamond is the largest gem-quality rough diamond ever found, weighing 3,106.75 carats (621.35 g), discovered at the Premier No. 2 mine in Cullinan, modern-day South Africa, on 26 January 1905. It was named after the chairman of the mine, Thomas Cullinan.