Akoya Pearls (by the strand)
18-in. strands, 6 to 6.5 mm
Round Tahitian Black Pearls
8 to 8.5 mm
10 to 10.5 mm
Euan McIlwraith reports on Scotland's endangered freshwater mussels
Despite being protected since 1998, Scottish freshwater mussels have been increasingly targeted by thieves who kill the mussel to get at a possible pearl inside.
Most don't contain a pearl and the discarded shellfish are left to die on the bank.
Following recent cases on the Spey, the police in Moray are calling for the public to help to report the lawbreakers, before the mussels become completely extinct.
Douglas Darling, a wildlife crime officer with Grampian Police, said: "What we found on the river bank was literally hundreds of these small pearl mussel shells, just discarded.
"The shells themselves, there's no way you would get any size of a pearl, if anything at all inside them. They're very young indeed.
"Action like this potentially would clear out one pool - one area of the pearls."
Scotland is a stronghold of the freshwater mussel with about 60% of the world's population living in our waters.
Freshwater pearls have been extremely sought after throughout Scottish history.
They're in the crown jewels in the vaults of Edinburgh Castle and it was even one of the reasons that Caesar invaded Britain.
Euan Lindsay, of romanscotland.org.uk, said: "Caesar in particular was interested in these and he was noted for actually weighing them in his hands before the financial transaction was done which was rather unusual for Rome's aristocracy and their elite to actually do that."
The selling of pre-1998 pearls is now strictly licensed and in fact there are only two jewellers in Scotland who are allowed to sell Scottish pearls.
But the mussel is still under threat - apart from illegal fishing, it's threatened by pollution and declining salmon stocks. So what's the connection with salmon in this whole process?
Ben Ross, from Scottish Natural Heritage, explained: "Without salmon there isn't any mussels, the larvae has to attach to the gills of a salmon or trout though it's not fully known why - but it may be that the salmon take it up the river and allow the species and the population to spread."
Even touching a freshwater mussel can be punished with a £10,000 fine, but it appears even that threatened sanction is not enough to deter the collectors.