Among ten ancient kinds of top quality inkstones, Duan is seen as the best. It has been caved for centuries in an area near Guilin. Songkeng (purple) and Mazikeng (dark grey) are the most famous; the latter because of fine traces, streaks or clearer zones that can appear from a stone to an another. In China, these "flaws" increase the price. Another strange Duanyan is emerald green and therefore called Lüduan. They all are living objects and will normally get a dark patina where ink is grinded. Wash with water only and wipe with a soft tissue. If ink has dryied in the corners, there is a special Japanese brush to clean them. NB : all Duan stones are different so the following pictures are given only as a reference.