Zini (purple clay, fired is usually brown-ish) exists as small amount inside Jiani ore bed. When Zini contains so much manganese that becomes dark dots after firing, it's called Hei Xing ni(aka Tie Xing ni, Hei Xing Sha). Each Yixing clay has a different ideal temperature of firing, different temp has to do with crystallization. Crystallization is important cos it will lead the clay no smell, no blot, better patina and better functioning. Zi ni's normal temp of firing is about 1150~1180C, Zini is rather easier clay to treat (Di Cao Qing for exception). Easy to craft, no worrying about firing. Higher temp-fired Zini turns grey-ish purple.