In which state of matter doesn't diffusion happen?

Diffusion is a physical process where molecules of a material move from an area of high concentration (where there are many molecules) to an area of low concentration (where there are fewer molecules). It happens in every state of matter, even in solid. In 17th century, Robert Boyle shown the process of diffusion in solids, with the example of zinc and a copper coin. Another known example is blurring of the colours of the glass or ceramic.