White metal is a mixture of several metals to attain a desired goal or need. The mixtures can consist of antimony, tin, cadmium, bismuth or zinc - and can sometimes cause a black or green stain, due to tarnishing of the metals (which is common in most jewelry).
Here is a great article about jewelry metals & why skin discoloration can occur: http://myhealthandliving.com/too-much-acid-in-your-body-will-make-your-skin-react-with-your-jewelry/
Most jewelry will tarnish (some more so than others), but everything that comes in contact with your jewelry, such as air, sunlight, natural body oils, dirt, household chemicals, food, etc - will cause tarnishing.
A few tips we recommend:
- Keep your jewelry away from extreme heat or sunlight
- Keep your jewelry clean and dry...removing your jewelry when cleaning, showering or swimming in a pool.
- Here is a quick guide on the care/cleaning of white metal as well: http://www.ehow.com/how_7968018_polish-white-metal.html