Hardened steel will look darker than unhardened at the same level of polish, so that might be part of what's going on here. Usually, quenching also causes less contrast in the hardened area, but that ingot I made with relatively high phosphorus did not seem to suffer from a contrast drop in the hardened area.
I'm not sure about the metalurgic reasoning behind how the light reflects off the surface, but since the pattern is due to the structure of the steel, there must be some metalurgy beghind it, intermixed with the reflection mechanics of the polish level.
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