Unglazed clay teapots are considered to the premium vessels for brewing premium Chinese teas, especially the teapots made of a unique clay called ZiSha (purple sand). YiXing area in the Jiang Su Province of China is where the ZiSha clay is found and mined, these clay teapots are therefore also known as YiXing ZiSha teapot.
Most of experienced Chinese tea drinkers would agree that brewing a pot of premium is such a dynamic process that every step counts, including the tea vessels used. The right vessel, combined with right tea, right water (temperature and source) and right brewing skills will bring the best out of a tea and the opposition will significantly affect the quality of the tea brew.
An unique nature of an unglazed clay teapot is that it allows the tea brewing process to breathe though the micro-cavities in the clay. (It is similar to preparing fresh vegetables, a pot with a tight lid and over stewing will kill all the nutrients and the flavours.)
Similar to all the premium Chinese teas, all ZiSha teapots are qulaity graded. The quality can be reflected in their sale prices, but the association is by far NOT linear. A careful selection with a bit knowledge will enhance your value/cost ratio.