Pyrite

Interlocking brass cubes with striated faces, sharp, sitting on a piece of dark shale. They wash out of the gill after heavy rain and lie on the shingle looking so deliberate that people assume somebody dropped them. Nobody ever believes they grew that way.

Pyrite is the best-known demonstration of the streak test there is, because the contradiction is so wide: a brass-yellow metallic mineral that leaves a dark, dirty line. Every key I own records the streak as greenish black, and I recorded it that way myself for a year.

Then I measured it. Off the shipped photograph, over the pixels that are actually deposit and not plate showing through, the powder has a chroma of 4.79 at a hue of 152° — which is green, in the sense that the arithmetic points that way, and which is well under the cut this register uses for having a hue at all. Held against malachite’s 11.19 it is not a colour, it is a lean. There is a very dark grey with a bias in it, and there is a phrase in a book that everybody including me has been repeating as though it named something.

I have left the description on this page as greenish black, because that is what the literature says and pretending otherwise would be its own kind of dishonesty, and I have put the measurement next to it. They disagree. The register sorts on the measurement, so pyrite sits off the circle with the greys, between galena and magnetite, ordered by how dark it is and by nothing else.

Which is the more useful correction this site has made to my own notes, and it is the reason the chroma floor exists rather than being a tidy-looking number in a file. Put the cut at 4 instead of 8 and pyrite is a green mineral. I have left the cut where it is and put the number on the page, so you can disagree with me precisely.