Answer: It's a closeup of my brake rotor, and those are hairline cracks across the surface. Not a good thing.
A few weeks ago, I took my car in to get the wheels balanced and rotated. The mechanic told me my front brake pads were looking thin and would need to be replaced soon. I'm rather picky about the pads used on my car; I always get Raybestos Brutestop high-performance pads, since they're one of the few brake pads I've tried that didn't overheat and crack. They're also kind of hard to find, and I had to go to 5 different auto parts shops before I could find any.
Lot of good that did me, since I couldn't find a mechanic who would install customer supplied parts. First I tried T. O. Haas, and walked out when they said they wouldn't install them. Then I tried Jensen, and although they wouldn't use my box of brake pads, they had that particular type in stock, so I went ahead and had them do the work. I would have tried to find someone else, but by then my brakes were squealing pretty loudly. I hope CarQuest will take back the pads that I'd bought.
Soon after they started work, the mechanic came in and told me the brake rotors were covered in heat fractures. The discs had overheated so badly that the thermal expansion and contraction had started to form cracks in the surface. Left they way they were, potentially the rotor could come apart under heavy braking.
He gave me three options: I could go with the stock rotors, which were relatively cheap but would likely have the same problem some point in the future, I could go with a thicker rotor which could absorb more heat or I could get a slotted rotor which would also help dissipate heat, looks sportier, but would take about a week to order. The stock rotors were $150 each, the thick rotors were $280 each, and the slotted rotors were $320 each.
I wanted the slotted rotors, since that's what I have in the rear, but I didn't have any way of getting home and it would have taken a week to order them, so I went with the thicker rotors. According to the mechanic, they could handle the heat better than the slotted rotors anyway.
I really didn't need this extra expense.