Osterrite is a sedimentary clayey rock, rich in silica and quartz, which has been subjected to an advanced level of metamorphism. The high pressure, combined with its depth, creates multiple cracks which, over time, become filled with minerals from the immediate environment. These parasite minerals which have left their original rock, occupy the cracks in the osterrite, restructure and eventually recrystallize in the vacant spaces. Due to the excessive wear of the terrestrial layer from the advance and retreat of many glaciations, these rocks emerge from the depths to eventually come to the surface where they are exposed to erosion (frost, wind, water etc.). The silica and quartz which have crystallised in the cracks are more resistant to this erosion than the host rock composed of petrified clay, and become more and more apparent over time. This is the origin of their rather bone-like appearance, due to the lamella and geodes that are sometimes found.