ˈneɪkriːəs

nacreous: pertaining to or resembling mother-of-pearl

Nacreous clouds are in the UK news at present, with multiple sightings in Scotland. There was an interesting divide in the BBC news coverage of the phenomenon this evening, with national newsreader George Alagiah intoning some twaddle about “forming at sunset” and “caused by refraction” in a sing-song voice, as if delivering a boring bedtime story. Whereas the BBC Scotland weather presenter, Gillian Smart, got the story right and had some nice pictures, too.

Nacreous clouds form in the low stratosphere, which is pretty high for a cloud. They’re present at all times of the day and night, but are more visible before sunrise and after sunset, when they are the first things to catch the sunlight, and the last to lose it, by virtue of their altitude. Their colours are due to diffraction, not refraction.

But this is a post about words, not natural phenomena.

Nacreous means “pertaining to nacre“. Nacre is the iridescent substance that lines many varieties of sea-shells, most notably those of pearl-forming oysters—it’s therefore commonly known as mother-of-pearl. The word comes to us from the Romance languages—it has analogues in French, Portuguese, Spanish and Italian—but its early origins remain obscure.

Nacre also provides the characteristic sheen on the surface of a pearl. And it’s interesting that a simple little world like pearl should also be a puzzle to etymologists. There are tentative links to Latin perula, a diminutive of perum, “pear”; or to a hypothesized diminutive pernula of perna, “leg of mutton” (from the shape of a mussel shell); or to  pilula, “globule”. Take your pick.

In Latin, a pearl is margarita, and in Greek, margarites.  Just as Pearl is a woman’s name in English, so Margarita is in Spanish. Margaret and Margery are its English-language equivalents. Margarita is also the Spanish word for “daisy”, though the connection between the pearl and the flower is obscure. The connection between the flower and the various cocktails called “daisies” is also obscure—at one time there was a Whiskey Daisy, a Gin Daisy and a Brandy Daisy, but the Tequila Daisy was the one that became most popular, and took the name margarita for itself.

Margarita also gave us the name for margaric acid, a mixture of fatty acids with a pearl-like lustre. Margarins were chemical derivatives of margaric acid, and margarine is a butter-like substance that took its name from the margarins, although chemically unrelated.

Something that looks pearly is margaritaceous, and something that produces pearls is margaritiferous.

Oyster comes from Latin ostrea and Greek ostreon. Something that resembles an oyster is ostracine, ostraceous or ostreaceous. The farming of oysters is ostreiculture.

An ostrakon (plural ostraka) is an archaeological find—a shard of pottery that has been used to jot down a note, something that was common practice in Ancient Greece. The Greeks called these pottery shards ostraka because of their curving resemblance to oyster shells. Votes were cast using ostraka, in particular when citizens voted for the banishment of one of their number. Such banishment was called ostrakismos—which gives us our word ostracism, meaning “exclusion”.