Bad head gasket on C15

So I just got interested in trucks and Diesel engines recently, and I came across this guy's channel. In the video I linked above he shows a C15 engine that kept overheating, and the driver thought that he had a bad water pump but the pump was actually good. After doing some digging they found that the driver was getting air bubbles in his cooling system due to a bad head gasket.

At 4:38 he shows the gasket. He mentions how there's a dark circle on the inside edge of his fire rings. My question is what exactly is this darkening, and what is it caused by? Part of me wants to say that it's discolouration due to heat distortion, and when he zooms in on the gasket (at 4:50) it almost looks like the fire rings are bent excessively or something.

 

I did, and I'm waiting for him to respond, that's why I asked.