Original Colt's had chrome BGC's. Why did Colt move away from this? Did this actually improve reliability?

IIRC some Colts had jeweled bolt carriers. Has anyone else seen this? I believe the idea was it would better hold an LSA type lubricant. Can anyone shed any light on this?

At some point there was a run of aluminum bolt carriers (during the 80's?). Was this ever a Colt part? What ever came of these?