Wrinkled banding

As Runzelbänderung is known under the polarizing microscope visible rhythmic canceling Chalcedonfasern transverse to the fiber direction of agate . The ribbons are a result of the twisting of the chalcedony fibers.

Quartz and its fibrous varieties chalcedony and quartzine are optically uniaxially positive. The higher refractive index is oriented along the optical axis . The optical character of the chalcedony fibers is "length-fast". This means that the higher refractive index and thus also the optical axis is perpendicular to the fiber axis. The twisting of the chalcedony fibers causes the optical axis of the chalcedony to rotate around the fiber axis.

The fibers always appear dark between crossed polarizers when the optical axis is aligned parallel to the polarization direction of the polarizers. The periodicity of the fiber twist is approximately the same for all fibers. This leads to regular dark stripes across the grain over the entire length of the chalcedony layers in the agate.