Set no. 2 - Grey Ridges

Images will appear in reverse order, can you figure out what you are looking at before you get to the bottom of the post?

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So what are you looking at?

It's the head of a socket head black oxide 1/4-20 machine screw.

Images captured using:
Nikon SMZ-1500
DS-L3-Fi2
Fiber Optic Ring Light