Fountain pen and marker is a good combination for doing concept art sketches. The pen is an inexpensive Waterman pen from an office supply store. You can fill it with any color ink that you want, as long as the ink is watersoluble. Markers can then give the sketch a touch of color without dissolving the pen line. If you want to melt the penline, you can use a water-based pen, like the waterbrush I’ve mentioned on previous posts.
This thumbnail sketch was made for Dinotopia: First Flight