Meerschaum is a soft, white mineral used to make pipes. When tobacco (or another substance) is smoked in it, the pipe begins to change that color over time. For your average pipe tobacco user, this color will be a dark mahogany or walnut shade. The color will start inside the bowl, spreading through use to the rim around the bowl and finally the outside of the bowl. Eventually, the pipe will be this shade in its entirety, making for a beautiful display specimen or a well-seasoned smoking pipe.
The only problem with pipes made of meerschaum is their fragile nature - a fingernail will dent or scratch the surface of the pipe, and dropping it will most likely break it. Fortunately, if the smoker only wants to smoke out of the pipe, as opposed to displaying it, a scratch or dent will not affect its ability to be smoked from, but it won't win any prizes as a display model.
I just noticed this node - it's an excellent guide on how to smoke a pipe.