What makes the Yixing pots special is the unglazed clay that they are made of. "Zisha" clay is found in the hilly areas around Yixing and no where else. Zisha is often referred to as purple clay, though it actually comes in other colours too (light brown to red). The clay is not chemically remarkable, consisting of quartz, kaolin, mica and iron oxide. It's the iron oxide that gives the reddish colour to the clay.
This clay is very porous when fired and is absolutely ideal for the making of teapots. Over time, the clay absorbs the flavours and oils from the tea. You should never scrub out your Yixing teapot, but only rinse it with cold water. They say that a well-used Yixing teapot can brew tea with hot water alone.
Yixing pots can be simple and elegant, or exotic and elaborate. Each pot is shaped by hand on a potter's wheel and marked by the craftsman (on the bottom and/or under the lid, and occasionally on the handle).
Teapots from Yixing have been popular in China since the Song dynasty (960-1279). It wasn't until the 1600's that tradesmen began spreading them throughout Europe and Asia. They were sought-after treasures, and often found in the homes of the social elite and even royalty.
The collecting of teapots is a popular hobby these days, and many collectors focus solely on Yixing teapots. The highest price ever paid for one was $70,000US at a sale in Macau. That pot wasn't even an antique. It was made in 1995 by the artist Gu Jingzhou.
Every true tea connoisseur should have at least one Yixing pot in his or her teaware collection.