Rock from road stall confounds the experts geologists

A piece of bright blue rock now puzzling mineralogists in the Natural History Museum may mark one of the rarest events on earth - a tourist getting the best of a deal with a Moroccan trader.

When Anna Grayson, a geologist, came across the fist-sized lump of rock at a roadside souvenir stall in Morocco 15 years ago, its seller assured her that it was lapis luzuli - well-known for its blue colour and exotic associations.

The stallholder sold it to Mrs Grayson for the equivalent of a few pounds, confident that he had passed on a piece of the relatively common mineral to one of the millions of visitors who buoy up Morocco's economy. But Mrs Grayson realised she had found something unusual, though her scientific training could not identify it. Until last year, she had left it at her home in Watford, labelled "unknown blue mineral".

Then a year ago, during National Science Week, she took it to the Natural History Museum, which was offering to identify mysterious objects. It proved tougher than most - and the experts soon found that the mineral's structure and composition, a mixture of calcium, iron, aluminium, silicon, oxygen and a pinch of sulphur, was not listed among the 71,000 officially registered minerals.

"We are still trying to work on the structure of the crystals," said Dr Gordon Cressey, deputy head of the mineralogy department yesterday.

"We are trying X-ray diffraction and scanning electron microscopes, which takes you pretty close to the atomic level. The crystals are very, very small. And it's so blue that when you look at light transmitted through it, it's like seeing a miniature stained glass window."

The team at the museum reckons that the mineral was formed by volcanic pressures inside the Earth, and is plentiful. "It must have been known in antique times," said Dr Cressey. "In fact, there must be a cliff face made just of this blue rock somewhere in the Atlas mountains [in southern Morocco, near Marrakesh]."

About 40 new minerals are discovered every year, but rarely in such large amounts; more often only pinprick-sized lumps are available for study. But they can resist analysis for years. Dr Cressey still recalls the case of "Parkinsonite" - a red mineral which needed 14 years' study to be pinned down.

At present, Mrs Grayson's stone is known simply as "the blue mineral". But although Morocco's traders may have lost out in selling it, they could still get the last laugh. To achieve full documentation of a new mineral, its place of origin must be included in the scientific paper - the discovery of which would almost certainly require the services of a local guide, at a rate to be negotiated.