The Ink Stone, together with the ink brush, ink (stick) and paper, are the four writing implements traditionally known as the Four Treasures of the Study. An ink stone is a stone mortar for the grinding and containment of ink. Ink stones are made from many types of stone, including Duanshi stone, from Duanxi, Guangdong. Duan stone is a volcanic tuff, commonly of a purple to a purple-red color. There are various distinctive markings such as eyes that were traditionally valued in the stone. Some rare forms of calligraphy ink come in solid rather than liquid form and are manufactured according to a formula which has remained unchanged for centuries. Soot, collected from small “soot rooms” where charcoal and/or wood are burned, is mixed with animal sizing, or glue. This mixture is then kneaded and compressed into special molds. The solid block of ink may then be painted or gilded. Oftentimes, the ink is laced with the scent of sandalwood or other perfumes.