title_body: What causes the stains around the cup when coffee is prepared with cafetiere (French press)?
upvoted_answer: I could think of two possibilities. First one is more common, I assume.
  
  
  In time, the cup calcifies. So, some irregularities causes an unpolished surface. This surface catches color easier than polished ceramic/porcelain. You may try to clean the cup with vinegar. Maybe then with some bleach to get rid of such calcification and organic irregularities. Give it a try.
  If the grounds you use includes finer grounds that can pass through the filter, you may end up with a muddier-than-normal French press cup. Those fine grounds may swim on top of the water and leave some trace. If this is the case, make sure about your grind size and consistency.