A wu shi plaque is a deliberately blank surface with only a cord hole at the top. The name is a pun: the characters also mean "nothing to report," so a plain plaque is a way of saying that nothing bad should happen. It is one of the few Chinese decorative forms that has no carving at all.
The jade here is nephrite, one of only two minerals properly called jade. The colour is a white with a faint green cast, and the plaque is polished on both faces with the edges left slightly softer so it does not cut the cord. Whether it is worn or kept in a box, the point is the stone and the plainness.