What happens, when coal seams burn? Not only in history but also in present times the coal fires had and have devastating effects on the environment undoubtedly. However, such processes led to a thermal influence of the rock surrounding the coal seams. The heat of the fire roasts the overlying sedimentary rocks. Most of them consist of clay minerals, which are thermal changed. Clay minerals were formed in dependence on the heat, the duration of fire, and by changing the composition of the sedimentary rock metamorphically. This process is called pyrometamorphism and the formed rocks are combustion pyrometamorphites. Different pyrometamorphites from a coal-mining region in Germany were characterized by chemical, physical, and mineralogical methods. The solubilities in alkaline solutions were also a main part of the investigations. Finally, the reaction behavior as supplementary cementing material was measured in comparison to other common substances. The results are exciting because some of the investigated materials have excellent properties regarding color, solubility, and reactivity. They can be characterized as a naturally formed metakaolin. However, there are limitations on its use. Not the complete material has such excellent properties; it depends on different factors. Additionally, it must be stated that the locations, where such rocks can be found, cannot be described as a normal quarry. On the other hand, this paper should be an incentive to search systematically for such rocks worldwide. The CO2 release in the past by coal seams burning can contribute to a CO2 reduction in the present and future.