Why is this iron black? Is it rust? No sir. Iron rust is reddish brown. Every reference book will tell you that iron is “silvery gray”. That’s a lazy and inaccurate catchall used with most metals which is only conditionally true for iron. One of the more interesting trivia bits about this otherwise rather uninteresting element is that its color is dependent on its size. It is in fact medium gray for anything down to the size of a pencil eraser. As the size of the piece decreases the shade becomes increasingly darker. Why, what’s going on?

The answer is rather technical but the gist of it is that when you see a color your brain is seeing light at a particular wavelength. A wave with a frequency of, say, 500 billionths of a meter (them’s some tiny waves!) appears green. Surf a 600 nanometer wave and it’s yellow, baby. Etc. When you see the light reflect off a piece of metal you’re usually seeing a whole bunch of wavelengths all at once from ripples to tsunamis and this “dirty” light is interpreted by your brain as white. The second factor at play here is the light intensity, or how fast those waves are coming in. Like a volume knob, the higher the intensity the brighter it will appear without messing with the colors. It is a continuum from white down to black with gray betwixt.

In the case of iron, as the size gets progressively smaller, the intensity of the light waves bouncing back drops. At a microscopic level the iron’s surface is able to grab on to more and more of the incoming light without reflecting as much of it. In other words, you perceive it as darker. Sometimes the electrons in the atoms directly mess with the light and do a bit of filtering of their own. On gold, for example, those light waves enter a quantum sluice of sorts that strips out most of the spectrum below 600nm meaning that only the yellow (and some red) gets bounced back to your retina, and you’re like, wow, so pretty!

Anyway, back to iron, the 1mm spheres here are small enough to rate as dark gray. Smaller than this and they’d be pitch black as the light rays get all gobbled up leaving very little to get back to your eyeballs.