The largest Scottish freshwater pearl found in living memory has gone on display at the National Museum of Scotland.

The "one-of-a-kind" Abernethy Pearl was discovered inside a mussel near the River Tay in 1967.

It is named after its finder, William Abernethy, known as the nation's last pearl fisherman.

Measuring 11mm in diameter and weighing 43.6 grains, the perfectly round freshwater pearl is the largest found in Scotland since the 1621 discovery of the Kellie Pearl, which is now thought to be on the Scottish crown.

It is believed the Abernethy Pearl could have been quietly growing in its mussel for more than 80 years before being found, meaning it was created during the reign of Queen Victoria.

Rachel Walcott, principal curator of earth systems at National Museum of Scotland (NMS), said: "The Abernethy Pearl is a rare and special specimen and I'm delighted that it's been donated to the national collection.

"It's a thing of beauty, but it also highlights the impact of the biodiversity crisis.

"Pearl fishing was once a very important export industry but the populations of mussels in Scotland have been decimated.

"We simply wouldn't have 80 years of undisturbed water in Scotland today for a pearl of this size to be created. It's one of a kind."

The Abernethy Pearl was donated to the nation by retired Edinburgh jeweller and NMS volunteer Alistir Wood Tait, who spent his 40-year career working with Scottish gems and river pearls.

He bought it at auction last year for £93,951, which the auctioneers said was a world record for this type of pearl.

Ms Walcott said the Abernethy Pearl is notable not only for its size but also its beauty.

"It's actually perfectly formed, whereas often river pearls are what they call baroque, a bit wobbly, and they can often be brownish or striped," she said.

"So, to get something that big and something that beautiful pinky-white colour, certainly in freshwater pearls in the northern hemisphere, is very, very special."

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Scottish freshwater pearls have long been a "precious resource". One of Julius Caesar's biographers claimed he invaded the British Isles because of the pearls that could be found there.

Scotland is home to about 50% of the world's freshwater pearl mussels but its populations have been devastated in recent years by water pollution, habitat-damage and overfishing.

The mussels also depend on salmon during the early part of their lifecycle, so falling numbers of salmon in Scotland's rivers are also putting the population under strain.

It is now one of the UK's most threatened species.

Ms Walcott said that between 1970 and 1998, when a ban on all pearl fishing in Scotland came into force, pearl mussels were becoming extinct at a rate of "two rivers or tributaries a year".

"I sort of think of them as canaries of our rivers," she said.

"These mussels are an indication of the huge threats coming from all sides at them, not to mention overfishing by humans."