I have a strange issue occurring in my shop. We have had a few rings brand new from the manufacturer with diamond melee passing a diamond test before exposure to a torch. After work is completed the melee don’t make it past sapphire on any of our testers. What is even stranger is that the ones that do pass have fluorescence and the ones that do not pass have none. Has anyone out there had issues like this?
Possible overheating, or flux coating?. Maybe try pickling in Sparex, for extended time (heated, well vented)? This last is a bit crude, but maybe try a slow brass brush in Foredom motor. It just sounds like a coating of some kind.