TIM wrote:zi = purple, sha = sand.
Sand is a better grade then clay = ni
Most of the yixing now-a-day is made of clay rather then sand. But vendors will still call it sha.
The darker one is a ni, not clay and it could be a Taiwanese or Chiu Chow wheel turned clay.
The terminology here is a bit different than in the west. Here, in China, the teapots are never called Yixing. Yixing is the city they are made in and it does not refer to the tea ware made there. Instead, all pottery made in Yixing are called zisha, whether it be tea pots, teacups, tea pets, or even statues, and no matter what clay is used.
Zini, Zhuni, Duanni, and Benshanlvni, to name a few, are all different clay types, but they all fall under the name of Zisha. No matter what the quality, a Zhuni teapot would be called Zhuni, for the clay type, and never calle Zhusha. Zhusha, is actually cinnabar used in making red lacquer.
I know in the West, a lot of people call them Yixing, but you would not hear that term here. Just a bit from the Chinese side of it for you.