Liesegang ring, in physical chemistry, any of a series of usually concentric bands of a precipitate (an insoluble substance formed from a solution) appearing in gels (coagulated colloid solutions). The bands strikingly resemble those occurring in many minerals, such as agate, and are believed to explain such mineral formations. The rings are named for their discoverer, the 20th-century German chemist Raphael Eduard Liesegang.