In the ethiopian province North-Shewa rhyolithic rock-formations are reaching the surface. These formations are the host-rock for the formation of lithophysae. These lithophysae are offering sometimes fine precious opal in their center.  Common opal and agates are not from any interest for the ethiopian miners. A very typical part of the pattern of an ethiopian thunderegg is the formation of “water-level-agate”. The colour of these african agates is dominated by bluish-grey tones with a white-lined banding.