German geologist who laid the foundations of the science of microscopic petrography (the study of rocks in thin section, based on the optical properties of constituent mineral grains). He was appointed professor (extraordinary) of petrography at Strasbourg in 1873 and ordinary professor of mineralogy at Heidelberg in 1878. From 1888 to 1907 he was also director of the geological survey of Baden.

In the 19th century the study of the optical properties of minerals was in its infancy, and the research of Rosenbusch was fundamental. His monumental Mikroskopische Physiographie der petrographische wichtigen Mineralien (1873; “The Microscopic Physiography of the Petrographically Important Minerals”) outlines the practical means by which rocks can be identified according to the morphological, physical, and chemical properties of their component minerals. He also described new instruments and techniques for studying the optical properties of minerals. Utilizing new methods and results of petrographic research, Rosenbusch wrote a companion volume, Die mikroskopische Physiographie der massigen Gesteine (1877; “The Microscopic Physiography of the Massive Rocks”), presenting a scheme for classifying rocks. Both works became classic contributions to mineralogy and petrography. He also wrote Elementen der Gesteinslehre (1898; “Elements of Petrology”).