Specifically, it is a special gem with extremely high value and energy. This is the object of argument between the main characters in the story, Su and Jun Mu Nian.
By chance, Sue collected a whole piece of mustard jade, which was later broken and given to Zhuang, and Zhuang gave the mustard jade fragments to Sue.
Generally speaking, mustard seed jade is an important object in this novel and an important element to promote the development of the story.