I'm thinking of buying a new car, and one of the options it has is diamond cut alloy wheels.
What's the difference between normal and diamond cut alloys?
There's nothing "wrong" with them, except they cost more, the finish doesn't last long, and you can only refurbish the finish a couple of times.
But don't let that put you off, if you really want them.
More information here. The fact that that company doesn't guarantee diamond cut finish, even though they will do it, might also be relevant!
I'd never heard of this before. Apparently a diamond cut wheel is simply one that has been machined (no diamonds involved, as far as I can tell) to be shiny. They get clear-coated after the machining.
It removes a tiny bit of material (if done properly). Perhaps your friend is trying to save you the embarrassment of driving around with shiny wheels. Just kidding, I have no idea how polishing a wheel could harm it (assuming it was done properly).
It's your car, if you like them, do what you like!
It is only about appearance and diamond cut is in style this year. The aluminum cast wheels are coated ( usually black) then a diamond , or equal , cutting tool trims off the high areas giving a black and silver wheel. Diamond ( or exceptionally hard) cutting tools are used because cast aluminum typically contains about 10% silicon ; the aluminum and silicon combine to make some very hard intermetallic compound which will wear away most cutting tools quickly.