Green color in a diamond is the result of the stone coming into contact with radiation. In natural green diamonds this takes millions of years, and usually happens when a forming diamond is exposed to alpha particles in uranium compounds or groundwater.
Alpha particles turn the diamond’s surface green, but if the diamond is also hit by beta and gamma rays it will take on a deeper green hue. Only a small fraction of diamonds turn completely green.
True green diamonds are extremely rare and only a few are well-known. The largest and most famous is the 40.70-carat Dresden Green, which was originally mined near Golconda, India in the 18th century. Diamond dealer Marcus Moses brought it to England to sell to the king, but the monarch declined to buy it, and in 1741 the diamond was sold to Frederick Augustus II.
It has been kept in Dresden ever since, except during World War II, when it was moved to Moscow’s Green Vault for safekeeping.
Another famous green, the Ocean Dream Diamond, is the only diamond certified by the Gemological Institute of America as Fancy Deep Blue-Green. Little is known about the Ocean Dream Diamond other than its size (5.51 carats) and the fact that it was mined in central Africa.
The Gruosi Green Diamond weighed 100 carats when it was mined in South Africa. The stone was cut and polished into a 25-carat cushion-cut diamond with a clarity described as “near flawless.” Owner Fawaz Gruosi, who purchased the diamond in 1998, had it set in a ring surrounded by smaller black diamonds.
Green diamonds are few and far between at jewel auctions. In 2009, Sotheby’s Geneva sold a 2.52-carat Fancy Vivid Green diamond – the largest Fancy Vivid Green diamond to ever appear at auction, according to the company’s records. The diamond, set in a ring, fetched $3.52 million.