JOHANNESBURG, Aug 28, 2007 (AFP) - A small South Africa mining firm says it has unearthed the world's largest diamond, but industry insiders are sceptical about the find.

Picture published on the South African web site Mining Weekly

A photo of the diamond has been circulated on the internet, showing a massive green-tinged stone sitting on a desk next to a cellphone to illustrate its size -- which is purported to be 7000 carats.

This is double the size of the current holder of the world's largest title -- the Cullinan diamond which was found in South Africa around a century ago.

"If it is what it is, it's very, very rare," said leading gemologist Les Milner from the Jewellery Council Labs. "I tend to be very sceptical about this kind of thing."

He said it was hard to tell from the picture, but that the stone -- found in the North West province -- was the typical octahedral shape of a diamond.

"Nobody has actually seen it to ascertain whether it is a diamond or not," he told AFP.

An online mining industry website, Mining Weekly, quoted a spokesman for the company claiming to have found it, as saying the stone was in a Johannesburg bank vault, and the company was consulting with lawyers.

However the spokesman would not give the name or location of the mine, beyond saying it was in the North West province.

A spokesman for the world's leading diamond mining company De Beers said he had not heard anything to believe the authenticity of the claim.

"I have no information about the reported diamond find but I have my doubts that it is real," the spokesman Tom Tweedy told AFP.

"The North West Province does not have big mines. Green diamonds are also very rare."