For photos that are, you know, mildly interesting

That's called a pride fork.

It's probably iron oxide. The layers on top of the steel are thin enough to cause iridescence, similarly to oil slicks on water.

Light does weird things (birefringence, interference) in thin layers of matter (thin meaning lengths approaching light wavelengths). I used to operate an ultramicrotome to slice resin into 50–70 nanometer thick sections, and the way I could tell they were the right thickness was their iridescent gold color. If the sections started cutting green or purple I knew I needed to make adjustments.