See the thread titled "Estate Pipes Reclaimed" for more before/after fotos.
Estate Pipes Reclaimed = Diamonds from Coal
Vulcanite, a common stem material, will oxidize and turn from black to brown over time. It is easily cleaned with some time and care. The sulfur used to make vulcanite announces itself during cleaning; often best your wife or loved one is not around during scrubbing - they make faces and doubt deeply the value of a pipe when they smell the smell.
Lots of tricks to get the mess off that vary according to the seriousness of the oxidizing and the appearance of nomenclature on the stem (imprints, etc.). Light stuff can be removed easily with an electric toothbrush and some baking soda or toothpaste; a soak in bleach plus some post-soak rubbing works well if the stem isn't imprinted. Lots of people swear by Magic Eraser but, having tried it several times I find the stuff wears me out. For most nasty stems I lean to sequential wet sanding with polishing papers # 4000, 8000 and 12,000.