The key
Every colour below was measured off a photograph on this site — the mean of the pixels in the sampled band that are deposit rather than bare plate — and nothing was adjusted afterwards. Which sector a specimen falls in is arithmetic on that measurement. It is not a decision, and two specimens a person would call the same colour do not always land together.
£18 a specimen · Read within a fortnight of arriving
On the circle
Ten of the fifteen carry enough chroma for a hue angle to mean something. They are shown here in the order the register uses.
Off the circle
Four fall below chroma 8. A hue angle computed from them is the arctangent of two numbers that are both nearly zero, which is noise dressed as a measurement, so they are ordered by lightness instead. Pyrite's streak is described in every key as greenish black; measured, the green is 4.79 chroma units against malachite's 11.19, which is a lean rather than a colour.
Neither
One left no deposit. Across the sampled band, 2% is the cut for there being a streak at all; quartz reaches 0.42% against a next-lowest of 6.53%, because above Mohs 6.5 the specimen cuts the porcelain rather than powdering on it. Nothing on this site declares that quartz has no streak — that is where the number came out. The swatch below is the page's own colour, which is what a missing measurement looks like when you refuse to invent one.
By sector
| Sector | Range | Entries | Members |
|---|---|---|---|
| Red | 345° – 45° | 3 | Cinnabar, Erythrite, Hematite |
| Yellow | 45° – 110° | 3 | Realgar, Limonite, Sulfur |
| Green | 110° – 200° | 3 | Malachite, Atacamite, Aurichalcite |
| Blue | 200° – 345° | 1 | Azurite |
| No hue | — | 4 | Pyrite, Galena, Magnetite, Rhodochrosite |
| No streak | — | 1 | Quartz |
The buckets are not even and that is the finding, not a flaw in the sampling. Powders are overwhelmingly red, brown, yellow and grey; a blue streak is rare and a violet one does not occur here at all. The empty stretch runs 272° → 23.8°, 111.8° wide, and the ground this site is printed on sits at 328.1° — 0.2° off its midpoint, inside the one sector the subject cannot produce.