In the Flagstaff House museum, Hong Kong, Dr. K.S. Lo's collection of these 3 Mings to Qings dynasty pots (above) are bench marks of what Zisha should be.
When Yixing collector addresses Yixing teapot, the first name is usually Zisha. So what is Zisha? I am not going to have a lengthy explanation or zillion of online links to this scientific and Yixing "experts" opinion. However, I am just going to show 2 of mine as an examples, sort of a visual identification in short.
Zi = Purple, Sha = Sand. In the old days, whatever is not purplish hue or sand like quality, it was not considered as Good Zisha. Specially if you see gold/sand sparkle inside, it's even more hard to come by.
Comparing it with the texture and looks of a Qing's Zhuni pot.
K. S. LO COLLECTION IN THE FLAGSTAFF HOUSE MUSEUM OF TEA WARE