The pearls are found in Highland rivers

The sought-after pearls have been found in Highland rivers for hundreds of years.

Scotland is home to half the world's population of the rare mussels which produce the pearls.

However, the molluscs appear to be dying out and it is thought that only 50 breeding colonies remain.

The rivers where the pearl mussels still survive are being kept secret to try to protect them.

Historical references

Pollution is thought to be partly to blame for the decline in numbers.

However, Scottish National Heritage (SNH) said illegal fishing for the mussels continued despite the species being given full environmental protection in 1998.

The conservation agency is now looking for historical references to the pearls to try to understand the pattern of their decline.

It said that some of the documents would date back to the 12th century.

Roman historian Suetonius refers to Julius Caesar's preference for British pearls as one of his main reasons for invading the country Iain Sime Freshwater advisory officer

Freshwater advisory officer Iain Sime said researchers wanted to discover to what extent the number of pearl-producing mussels had changed.

"We were trying to look at the state of the pearl mussels to try to compare that with as far back in historical documents that we could go," he said.

"We are mainly interested in the decline in the last 100 years because records become more detailed and we assume that stocks in Victorian times were still relatively healthy.

"It is really putting them in their historical and human context rather than their conservation significance."

The findings will allow SNH to understand the creatures' habitat needs and target key areas for conservation.

Crown jewels

Freshwater pearl mussels can live for more than 80 years, but it takes them at least 10 years to reach maturity.

One specimen found in Aberdeenshire was used in the Scottish crown jewels.

"The oldest reference in Scotland tells how Alexander I, King of Scotland, was said to have the best collection of freshwater pearls of any man living at the time," said Mr Sime.

He added: "Writing in 55BC, Roman historian Suetonius refers to Julius Caesar's preference for British pearls as one of his main reasons for invading the country."