The biggest diamond ever may have been found in a mine in South Africa.
The company that owns the mine says it has found a diamond that is twice the size of the world's current biggest.
In photos the diamond looks a bit green and is around twice the size of a mobile phone, but diamond experts aren't sure that it is real.
One expert said large diamonds aren't normally found in that part of South Africa, and diamonds with a green colour are very, very rare.
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A spokesperson for the leading diamond mining company, De Beers, said that if the find were genuine it would be the stone of the century.
Diamonds are measured in something called carats, and this new gem is reported to be around 7,000 carats.
The biggest diamond ever found so far is called the Cullinan diamond - which is part of the British Crown Jewels - and is measured at more than 3,100 carats.