What is an Irish pearl?

Dear Mr. Alice:

Top of the Mulligan t'ye! Can ye be tellin' me what be an Irish pearl? I am the missus of the leprechaun who hoards that legendary pot o' gold at the rainbow's end. Himself is thinking o' expanding his inventory & heard tell of an "Irish pearl." His jeweler friend couldna tell him, yet there you are: Himself has read it in an English book written in 1950! It tells of a brooch "imbedded with an Irish pearl"! Meself bein' a Margaret, & that meanin' "pearl," I'd be longin' to ken meself. A thousand shamrocks on ye, & may ye find the end of the rainbow opposite of ours & belongin' to our competitor!

— Maggie O'Leprechaun, the End of the Rainbow

Letters from little people usually go straight to the research elves. But they earn pocket money this time every year by spray-painting each other green and hiring out for St. Paddy's parties. At the moment, they're taking their post-celebration turpentine soak, so I guess I'm on my own here. By the way, I have specialists decoding that last benediction. Grandma Alice is sweeping up the thousand shamrocks. That's no problem. It's the "opposite end of the rainbow" stuff that has us leery. Or O'Leery, if ye prefer. What's the opposite of a pot of gold? I don't even want to think about it.

Michael Viney writes on ecology for the Irish Times (and not in dialect, I'm glad to say). He was plain gob-smacked to hear from this side of the pond on such an esoteric subject. But yer longin' to ken; Mike's glad to oblige.

In certain rivers in Ireland dwells a freshwater mussel known to science as Margaritafera margaritafera, commonly, the pearl mussel (as opposed to Ireland's pearl-less duck mussels and swan mussels -- which produce ducks? swans?). The slow-growing bivalves spend their formative years in the gills of obliging fish. When they're heavy enough to fall out, they settle into dark crevices in river bottoms, where they can live for as long as 100 years. If an irritant gets inside the shell, they spin layers of nacre around it, just as an oyster would. In the case of M. margaritafera, the nacre can be any color from white or lavender to muddy blue-green or pure black.

For you Scots out there, go back to the beginning of this story and substitute the word "Scotland" for "Ireland" and "Scottish" for "Irish," and the facts are just as true. The shellfish are found in other parts of Britain and Europe. In 1969 a Scotsman (with the luck of the Irish?) harvested a large lavender pearl near Aberdeen, now worth about a quarter of a million U.S. bucks, and the great Scottish Pearl Rush of '69 was on. It was a typical goose/golden egg scenario. The mussel's now an endangered species, and Margaritafera molesting is illegal everywhere in Europe. Maggie maggie is still poached, but she hasn't been harvested legally in 20 years. Marauding Romans and centuries of rich locals have favored Irish pearls, but they have never been sold much on the international market, despite the fact that the pearls are all natural, not cultured, which adds to heir value.

So, Maggie, if O'Hubby can find himself a big lustrous black Irish pearl, the O'Leprechauns'll be in clover. Or maybe in the Paddy O'Wagon, if he is caught.