Purple of Cassius is a colloidal solution of gold that is typically produced by reducing gold chloride in the presence of tin chloride. The colloidal gold particles are dispersed in a liquid, giving the solution a characteristic purple color. Gold nanoparticles, when prepared in a colloidal form, can exhibit unique optical properties due to surface plasmon resonance, which is responsible for the purple color. This colloidal form of gold has been historically used in glassmaking and is also significant in the study of nanotechnology.
