Almost all metals will darken over time, especially as they interact with your skin and the open air. The exceptions are generally gold and the more precious metals. Here's some of the basic things you can do to keep your jewelry looking fresh.
General Tips for All Metals
- Don't wear your jewelry in water. The shower, hot tubs, pools, the ocean -- they all can have different effects on your jewelry, especially water with chemicals in it (chlorine in pools, sulfur in hot springs).
- Avoid chemical jewelry cleaners. Use soap, water, and a soft tooth brush to clean your pieces instead. If you need heavier duty cleaning, bring them to a jeweler.
- Sunshine polishing cloths are your friend. They polish almost anything wonderfully.
Silver will tarnish over time as it's exposed to air. The oils on your skin retard this process, so the more you wear your silver jewelry, the less likely it is to darken with tarnish.
Storing your silver jewelry in closed containers (boxes with lids, ziplocs) will go a long way towards keeping tarnish off when you aren't wearing it.
Because brass is an alloy made up of a few different metals, it can also tarnish over time. Lots of people like the patina brass takes on over time; if you want to get rid of it, polishing with a polishing cloth occasionally will do the trick.
Brass can also react with your skin to produce a darker tone or green color. It doesn't mean you're allergic -- it's a natural side effect of the metal interacting with your skin. Temperature, humidity (sweating!), and hormones all factor in to whether or not the discoloration happens. If it does, it's nothing to worry about. It will wash off with soap, water, and a washcloth.