We have a big one made of white marble, which my mom lugged back from a trip to India...but I've seen very similar sets sold at most Indian stores on Devon. Ours is a beast, about 8" in diameter (6" bowl diameter), 6-7" pestle. The bowl is slightly rough (not nearly as rough as the volcanic rock ones), and the end of the pestle is slightly rough...just what you need for grinding spices, without large pits for your ground spices to get stuck in.
The big aesthetic problem I have with ours is that the white marble stains really easily...no matter how much we try to clean it, the bowl is brown from years of cumin grinding. If I were to buy one now, I'd go with a granite one (which also should be widely available on Devon...looks like around $40 for an 8" diameter set is about the norm). If granite mortars are anything like granite counters, it should be more resistant to staining than marble (especially white marble).