Though possibly created by the drying or freezing processes, these cracks indicate mechanically weak regions or materials with different thermal expansion coefficients inside the gastrolith structure. The cracks also partially follow the positions of the darker layers in the middle of the gastrolith in transmission light microscopy images (see Fig. 2D). Here they seem to be associated with the interface between the darker layers and surrounding transparent regions, as will be even clearer by chemical analysis discussed in the following paragraphs (see Fig. 2B). As the dimensions of the cracks are much smaller than the darker regions in light microscopy, and only a part of these regions are associated with cracks in SEM, this excludes the cracks as being the cause for this difference in opacity.