Chihuahua Blue Agate is a  mysterious material. The origin of these stones is described by Brad CROSS in its book “The agates of northern Mexico, 1996, p. 62, 63, pl.19” as “from  northern Chihuahua”. These beautifully blue colored, weakly banded agates are so-called “Thunderegg cores”, i.e. lithophysae agates without lithophysae. These are completely wheathered and gone. However the core, which consists of penetrating discoidal disks, remained. Cut in the correct direction these cores are forming star-shaped cross sections.