Other metallic pens (most notably gel pens) can also be used on polymer clay.
With the popularity of scrapbooking and other paper crafts, there are all sorts of fancy pens on the market, including a rainbow of pearly and metallic ink pens. Most of these can be used to embellish the surface of your polymer clay creations. Just be aware that some ink may bleed into the clay over time, particularly if the clay is light-colored or somewhat translucent. Other inks may not dry completely until they have been baked. Since some inks may not be waterproof (even after curing), you might want to seal your project once you're sure the ink has dried. If you have problems with a certain brand of pens, there are probably others that will suit your needs.