Bruneau Canyon in southwestern Idaho is offering to the collectors a very fine jasper with an unusual structure. This structure is called by the american collectors “egg-shell pattern”. This “orbicular structure” (the scientific term) is caused by enrichment of iron-oxid pigmentation in prefered parts of the jasper.
This special jasper occurs in thundereggs, similarly as the Willow Creek Jasper. In Bruneau Canyon the thundereggs are reaching however only the size of up to 25 cm. If gathering of good pieces at the surface was possible in the starting time - the 1960ies - meanwhile the material must be blasted out of the rhyolitic hostrock.
Only a few claims are working nowadays. All this points are causing the exceptionally high prices paid for Bruneau jasper today.
Hans gamma is showing on his website worldofjaspers.com beautiful specimen of this locality.