THE passion of American mining experts for a rare mineral has led to the survival of the last fluorspar mine in the North Pennines.

From the tiny mine, perched 60ft up a cliff-face in a derelict quarry, the Americans are extracting tons of the quartz-like mineral once vital to Britain's steel industry.

Ninety-nine per cent of the fluorspar is washed and polished then shipped out to the US to be sold mainly in gift shops throughout the country.

But a precious one per cent of the mineral, which changes colour from green to blue when exposed to daylight, is snapped up by collectors.

One piece of this - the size of a man's fist - can fetch up to £1,000.

"This is the only place in the world where this kind of fluorspar can be found - it is absolutely unique," said geological engineer Cal Graeber, from San Diego, California.

He and his three partners "inherited" the mine - at a secret location in Weardale, County Durham - from two Cumbrian cavers and mineral collectors, Lyndsay Greenbank and Mick Sutcliffe.

The two men discovered it during one of their searches for rare minerals and began working it more than 20 years ago.

While on holiday with his wife, Mr Graeber met the two cavers, who showed them their mine. When they retired, he and his UK Mining Ventures company took it over.

The Americans lease the mine from a local land owner and pay the Church of England for mining rights. Through its centuries-old association with the Land of the Prince Bishops, the church still retains mining rights in many parts of Weardale.

So far, said Mr Graeber, they had made no fortune.

"The odds of any mining venture succeeding are slim," he said, "and when you are just mining for ore for collectors, it's a real gamble.

"You have to invest a lot of money in the mining business before you can hope to see even a nickel back, but in the next year or so we hope to go from the red into the black."

Mr Graeber said they had invested "hundreds of thousands", not thousands of pounds, in their Weardale venture.

Only a handful of Americans work on the extraction of the fluorspar, which is then transported to a washing plant before being crated and sent off to the US. Some local people are employed during the two months the Americans spend with their families in Weardale during the summer.

Jonina Pogue, another partner who handles much of the marketing of the ore, said the one per cent they sell to collectors was "very beautiful and almost chameleon-like" when it changed colour.

She said that when they first displayed it at the world's biggest gems and minerals exhibition in Tuscon, Arizona, everyone was "gob-smacked by its sheer beauty".

When he first saw Weardale fluorspar Mr Graeber had a vision about finding his El Dorado.

That vision still has to be realised