The mussels are killed to get at the pearls

Scottish Natural Heritage carried out spot checks and found more than 250 pearls at three shops in Edinburgh.

Legislation was introduced in 1998 making it illegal to fish for or sell freshwater pearls.

Scotland holds half the world's population of freshwater pearl mussels, although there are only 61 known breeding sites left in the country.

Pearl fishing involves killing the mussels, most of which are without pearls.

Under the law, it is illegal to fish for pearls and to sell or advertise them even if they were collected before the ban was enacted.

It is illegal to sell freshwater pearls

John Ralston, licensing officer at SNH, said: "Our freshwater pearl mussel is in serious danger of becoming extinct, but we are determined to not let this happen."

The freshwater pearl mussel is deeply embedded in Scots culture, particularly as the famous Kelly pearl is thought to be part of the Scottish crown jewels.

The species is known to have been exploited in Britain since Roman times and was fished extensively between the 16th and 19th centuries.

As a result numbers declined steeply, so the trade became unsustainable and shrunk to a relatively small-scale activity.