"Do you know of a resource that tells you which gems/semi-precious that can/can't be tumble polished with stainless steel? I am looking for a general resource, but specifically moonstone."
I don't know of a definitive resource and I don't really see how there could be one because a lot depends on the quality of the stone. Low quality stones that have cracks and fissures or are chalky aren't as likely to take the millions of tiny pings of tumbling as well as higher quality, more solid stones. And that's just one example of a difference...
If you've read our Cleaning and Polishing page reference above, you've seen the bit about the mysterious darkness. Yellowing is the first stage of that darkness, just like it's the first stage of tarnishing. If you get yellow, brown, grey or black on your metal and you've followed the directions for rinsing the tumbler and shot, adding clean water and a lot more Dawn, given it another tumble and it still hasn't gone away, you need to do two things.
We highly recommend the use of a rock tumbler and stainless steel shot for cleaning and polishing jewelry. We favor that method so much that most of this page and all the photos are devoted to explaining in detail exactly how to do it, despite the fact that we don't sell tumblers or any supplies for them. We hope you find the information helpful and delight in the relentless shine of the finished product as much as we do.
If you've ever polished the family silver, you've had your introduction to cleaning sterling by means of chemical compounds. There are sprays and dips intended for use on jewelry, as well, but because we prefer to keep our chemical use to a minimum, we have no experience with them. These are the methods we've used.
A mixture of lemon juice and salt or white vinegar and salt is a very effective solution for cleaning copper. You might have seen half a lemon salted and used to scrub copper bottom pots. For jewelry, dissolve a tablespoon of salt in a cup of lemon juice or vinegar (measurements are fairly arbitrary and can be adjusted as you see fit) and keep the mixture in a jar with a lid. Drop the jewelry in the jar, swish it around for a moment, then remove, rinse, rub it all over with baking soda to neutralize it, then rinse it again. However mild, lemon juice and vinegar are still acids and you don't want to leave acid sitting on your jewelry. If it isn't neutralized....