Physical and chemical properties of geomaterials are central to our understanding of all geological process. Yet, at high stress and temperature as well as in harsh chemical environments, measurements of many important properties are challenging. In the experimental group CLEAR in Section 3.4 we use spectroscopy-based techniques to better constrain physical and chemical properties under extreme environmental conditions. To this end we employ well-established optical methods but also develop new time-resolved techniques that offer insights into the properties of crystals, rocks, and melts. Our work enables a better understanding of core geodynamo, mantle convection, volatile recycling via subduction, and differentiation of the magma ocean in the deep past.