Duanni is also rare, coincidently when co-exists with Luni and Zini. Most Duanni today however, is from Tanxi or Hufu or intentionally mixed with Luni and Zini. Zima(sesame) Duanni has many dark dots that resembles sesame. Bai Ma Zi ni is very similar to Luni after firing but not smooth and lots of grits.
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. Minimum temp for Luni is about 1180C, lesser temp will lead Luni famous 'blot' or 'crack'. I've seen some sellers recall Luni pots caused by amateur firing. Luni is a clay that has no plasticity, so firing is very complex and requires experience. So to solve this complexity, potters make 'Duanni' by mixing with Zini. Low-temp fired Luni is beige and sees a hint of green after 1200C (Duanni as well).