OMG you guys are joking right?
I'm laughing my ass off at this. Especially the wear indicator comment... That's like, if your engine starts malfunctioning on the freeway, explosive bolts blow the wheels off the car to immobilize you while OnStar calls for help.
The surface is machined but the groove I'm talking about is huge, it's like 1/16" wide by about 1/32 deep. It's wide enough I can stick a dime into and half as deep and it's black. So I have a silver surface with a black ring going around it.