Diamonds may be a woman's dream but this one would break her neck if put on a necklace: it has the size of two fists, 7,000 carats and doubles what was till now the world's largest known diamond, the Cullinan or "Great Star of Africa", found in South Africa in 1905.

The new one has been found by a small mining company also in South Africa in an unknown location, but check is still required. Security is tightened around the stone.

The stone was stored in Johannesburg, where it would remain "until we calm down and decide what we are going to do", said Brett Jolly, a spokesman for the mining firm Two Point Five Construction.

By now, neither the South Africa Diamond Board, nor the Diamond and Jewellery Federation, the trade body, confirmed the authenticity of the new discovery and the only current proof is a mobile-phone photograph of the stone, presenting a smooth gem with a rare green tinge.

The first ones checking the discovery reported an industrial grade, but a geologist will analyze the stone the next days. The huge diamond could be cut into a piece of spectacular jewellery. Cullinan had 3,106 carats when discovered on 25 of January 1905 and was cut in two smaller diamonds, being part of the jewels of the British crown.

The specialized companies still refuse to believe the new stone is a diamond. The mystery is increased by the fact that the huge gemstone is still under lock in a Johannesburg vault, without a test, while it would take an experimented gemologist just 20 minutes for a diagnosis.

"Without seeing the actual stone I couldn't say 100 % whether it is a diamond. The form of the stone is that of a diamond. It just looks so perfect, which is amazing. People tend to be very skeptical at the moment. There is no actual proof as such yet, it's all in the air." said gemologist Les Milner of Jewellery Council Labs in Johannesburg.

If the new stone proved to be real, it would be "the eighth wonder of the world".

"Nothing's impossible. When they found the Cullinan, they thought that was amazing."