skrap1:
Peckinpahhombre hit the nail on the head. Vulcanite naturally oxidizes, and it is sulfur which causes the brown-to-green discoloration.
I have found that Bon Ami brand cleanser does a good job of removing this oxidation, and it does not scratch the rubber material. Simply use a dampened piece of paper toweling to which a liberal amount of Bon Ami has been applied, thoroughly scrub the stem (taking care to avoid any stamped logotypes or nomenclature), and then rinse thoroughly.
Depending upon its appearance and your personal preference, you may then gently wet-sand starting with 600-grit wet/dry paper, and use successively finer grits until you have a smooth, even finish. For maximum gloss I buff (using a bench-top buffer) gently and at slow speed, and dress the buffing wheel with red rouge. Again take care to avoid logotypes and nomenclature.
I hope that this information is useful to you.
Good Luck, and Good Smoking!