Alloy idlers worn... look like they've been sharpened!

[Monkeysmate]

I noticed one of my lads cars sounding slightly rough, but still running OK.
I noticed that the alloy idlers appear worn and look like they've been sharpened...
Can anyone tell me why this may have happened?
I've gone back to the white plastic ones for now...

Also, can anyone tell me what the black plastic idlers are made of? Are they any good? I found some in some spares...
Thank you,
Tim