experiment. High sulfur and acid content= lovely blackened silver. It also happens on very sweaty acidic skin. My sterling silver at fist looked gold, and now has a lovely darkened coppery tone. No problemo to fix:
baking soda! Dry powder or a paste with water.
I've even seen gold tarnish depending on the VOCs in the environment (think of the time you found a loved one's long forgotten ring in a drawer that had turned an unusual color.) Very diluted ammonia with mostly water can be used, or the baking soda trick again. Failing that, your friendly local jeweler can certainly buff it back to normal!
Gemstones are of 2 schools- the tough and the fragile. The tough are without a doubt diamonds, sapphires, rubies. Use 5 parts hot water with 1 part ammonia... soak and scrub with a toothbrush to soften and remove any clinging gunk. Other stones... best bet would be to clean then with just hot water and a soft toothbrush. Nothing acidic. Nothing abrasive. Water softens the debris without being possibly invasive. You especially want to try to keep very transparent stones clean - like quartz (amethyst and citrine) and aquamarine, as debris immediately dulls their beauty. Hot water soak and toothbrush, easy enough for anyone to regularly do.
Remember, when scrubbing your jewelry, do it in a controlled situation so any stones dislodged are retained. Happy cleaning