Jeweler's Rouge is what I use with my Dremel. Unfortunately, the stuff I have right now is really, really, soft, and tends to just fly right off the felt pad I use.
As for most valuable, I would have to say, the jewelry that I've recovered.
Interesting, at least for me, is the coins I get. I primarily want to find old coins. Earlier this year, I found an 1873 Seated dime in extremely good condition. What's really weird, is it was practically laying on top of the ground. I found it out in the woods, in the center of a trail, a half-inch under the surface. As it is, this dime is worth between $35 and $130, depending on which variation of the "3" is in the date. To bad it isn't an 1873CC "with arrows" (CC is the mint mark for Carson City, and the "with arrows" denotes some arrows near the date, indicating a slight change in silver content). That dime starts out at $3K. Here's a picture:
The strangest thing I've found (and I've found several), is a fired bullet on school property. It always makes me nervous when I find one, as it means that somebodies kid could have been hit.