Hi S.,
Sorry, I would have answered this sooner, but your question just showed up in my email today after you had already gotten responses to it. I worked as a bench jeweler and metalsmith for over 15 years before I had my daughter and I've seen a fair amount of that type of oxidation in 10K gold, particularly around solder joints and sizing bars. It likely did occur as a reaction to contact with minerals or chemicals in water. The good news is, it's on the surface and should easily be buffed out by a jeweler. You really won't ever be able to return the color to normal on your own at home. Toothpaste is mildly abrasive, so while you may have managed to strip some of the oxide off, you probably scratched and dulled the surface of your ring, making it look dingy. A professional jeweler can return it to its original shine. If there are only diamonds in the ring, and no other stones, it's impossible to damage those with any household cleaning products. Once you've had a jeweler buff your ring for you, always take it off before doing dishes, laundry or swimming in a chlorinated pool. The chlorine in detergents and pool water will break down the copper alloy in gold jewelry, eventually making pieces very brittle and will cause stress fractures that will make it irreparable. (10K yellow gold is only 41.7% gold metal, 14K yellow gold is only 58.3% gold metal. The rest is a mixture of copper & silver.) Anyway, I leave a little dish next to my sink that I put my rings in while I'm cleaning house. In future, if you want to clean your fine jewelry at home, warm up a little Windex and dilute it with some water, soak your jewelry for about an hour, then rinse it under warm water and wipe clean with a paper towel. It won't polish anything, but it will strip off the cosmetics or hair products that have a tendency to stick to and discolor fine jewelry. Jewelry cleaner is just a mixture of ammonia and metal neutral detergents. I used it in my ultrasonic cleaner for years and it's safe to use on almost any gemstone but pearls, opals or cameos. I would offer to polish your ring for you, but I'm all the way in South Florida. :) I hope this info helps.