I bumble around with the green stem problem and do not have a bench polisher, which maybe I should, but I don't. So currently my best "cure" for vulcanite stem oxidation is this --
I have a few small sanding pads from a paint store. One side is light abrasive, the other is spongy. They're flexible, can be wrapped around a dowel, and they can be used wet or dry. After wet sanding, I dry the mouthpiece and it will look scratched and dull. Remain calm. Then I take just the slightest dab of mineral oil on my fingertip and rub it into the mouthpiece. Next I run a pipe cleaner through the mouthpiece, bend a little hook into each end and hang it on something handy to dry. After a few hours, the bit will look glossy and black. Nonetheless, I wipe off the excess mineral oil, which dulls it a bit, and polish with Brebbia or whatever is handy, including pipe bowl polish or sunscreen Chapstick. Repeat the polishing and, over time, your stems will look pretty spiffy, though not quite Mercedes Benz Black.
I'm wondering if Obsidian Oil is the same thing as mineral oil, which, in addition to its well-know uses, is also an anti-oxidant for high-carbon knives.
I have used the above technique, gotten to the post-sanding stage, rinsed the stem off, dried it and gotten a result of a not-entirely unattractive, uniform, smooth greenish hue. (I have no idea . . .) However, after a second sanding and app of a teeny dab of mineral oil, the stem turned back to black, or at least a very dark grey.
The whole process, drying time excepted, takes no more than a leisurely ten minutes.
CAUTION: Mineral oil, even a little bit of it, runs, and I try to avoid getting any of it on the tenon or into the pipe's tenon hole. The "hooked pipe cleaner" allows me to reverse which way the mineral oil flows if it threatens to get on the tenon.