Stromatolites can be found in the sandstone sediments in the area of Bolanden. These are fossils of blue algae colonies, which can reach a size of up to one metre diameter. At the beginning of a large colony of such kind is always a delicate algal film that flows trough the water and sometimes builds up on an object near the water surface. In the present case, this was probably a small stem of a tree fern. In the course of Fossilisation, only a hole remained of this stems in the midst of a chalky stromatolite. In the course of later Silicification an agate was formed here.
Slightly smaller in diameter, but bleached by the rearrangement and embedding in sediments and more contrasted, a similar specimen from Arenrath presents itself here.