While hard materials such as agate and amber are difficult to carve aesthetically, soft stones such as Ch'ing-t'ien stone, Ch'ang-hua stone and Shou-shan stone are liked by seal carvers because they offer scope for calligraphic knife-work.
硬石不堪入刀，不易表現美感，軟石比銅、玉易刻，且可看出刀工筆法，為後代篆刻家所喜。