Aurichalcite

Feathery tufts of pale blue-green in a shallow cavity, made of needles fine enough that touching them is destructive. I collected the rock rather than the mineral; there was no way to take one without the other and no way to clean it afterwards. It sits in a box on its own.

A zinc-copper carbonate, and the colour sits exactly where the chemistry says it should: between the copper greens and the copper blues, at 187°. Before it was streaked the register had a hundred and thirty-six degrees of empty circle in the middle of it, between malachite and azurite, and I had begun to think there was a real reason for it — that carbonate powders simply did not go turquoise. There is no such reason. The gap was the collection being small.

That is a thing worth recording plainly, because it cuts against the flattering version of what this site does. An arrangement by measurement looks objective and it still only describes what has been put into it. Every empty stretch in the key is two claims at once: that the district does not produce that colour, and that I have not found it yet. Only one of those is about mineralogy.

The gap that remains after this entry — from azurite round through violet and magenta to the reds, a hundred and eleven degrees of it — is the one I have never once threatened to fill. Powders do not go that way. If a specimen ever turns up here that streaks magenta, this whole site will need repainting, and I would take that trade.