False colour
The pigment that makes diatoms golden is named, if you follow it far enough, after eye makeup.
Diatoms are not green. They photosynthesise, they carry chlorophyll, and they are nevertheless a sort of warm olive-brown going to gold, because sitting on top of the chlorophyll is a pigment that absorbs in the blue-green part of the spectrum - which is the part that gets furthest down through seawater - and hands the energy on. It’s one of the most abundant pigments in the ocean and hardly anyone outside the field has heard of it.
It’s called fucoxanthin.
The back half is easy. Greek xanthos, yellow. Same element in xanthophyll, and in Xanthippe, who was presumably fair-haired before she was anything else.
The front half is Fucus, a genus of brown seaweed - the leathery, bladdery stuff on rocks at low tide. Named in the early eighteenth century from Latin fucus, a reddish seaweed or rock lichen. So far this is an unremarkable pigment name: yellow-of-the-seaweed. Fine.
Except that fucus in Latin doesn’t primarily mean seaweed.
A red dye was made from that lichen, used on wool. From there the word slid to mean the dye itself, then rouge, then the general Latin term for the paint women put on their faces. And then, because that particular metaphor is apparently irresistible in every language, it slid once more and came to mean disguise, pretence, a false gloss put over something. Roman writers use fucus for a specious argument the way we’d say varnish or spin. It arrived in Middle English carrying the deceit sense before it ever arrived carrying the seaweed one.
Behind the Latin is Greek phykos, which has the same double life: seaweed, and also the red cosmetic made from it.
And behind phykos, probably, is something older still. The usual proposal is a borrowing from a Semitic language - the comparison is with Hebrew pūk, a mineral eye-paint, the powdered stuff people around the eastern Mediterranean used to darken their eyelids. It turns up in the book of Kings, where Jezebel puts it on before going to the window to meet her death, which is not a scene anyone was likely to forget. If that trail is right, then at least one etymologist has argued that makeup is the original sense and seaweed is the derived one. The word would have meant the paint first, and then the organism you made the paint from.
I want to be careful here, because this is the point at which etymology becomes storytelling. Ancient loanwords are hard, the Semitic connection is a proposal rather than a settled fact, and the direction of the shift is a judgement call about which sense came first when both are attested early. It’s a strong candidate, not a certainty. But it’s a good enough candidate that the Greeks themselves seem to have used the two meanings side by side without embarrassment.
Which leaves us here: the science of algae is called phycology, from the same root. The study of seaweed is named, if the trail holds, after eye shadow.
There’s a modern coda that I didn’t go looking for.
The images of diatoms that circulate - the ones people share because they can’t believe something that small is that ornate - are mostly electron micrographs. An electron microscope doesn’t see colour. It doesn’t see light at all; it counts electrons and produces a grey image, and every one of those tawny golds and lilacs and teals was applied afterwards by a person with a mouse, to make the structure legible or simply because it looks better.
The technical term for that is false colour.
So the most widely seen pictures of an organism whose defining pigment is named, at some remove, after cosmetics, are themselves painted after the fact. Nobody planned this. Words don’t have intentions and I’m well aware that finding a pattern in four thousand years of borrowing is exactly the kind of thing a person does when they’ve been staring at one subject too long.
Still. It’s a good run.