Around Tsugaru in Aomori Prefecture, there are several finding-sites for Thundereggs, which differ widely in their interior. Sometimes even the outer shell is missing and to recognize the origin of that specimen from an acidic volcanic rock is difficult. Fortification-agates and agates with gravitational-related banding ("Uruguay agates") occur.

Hideharu Yamada is offering a lot of information about japanese agate and others on his website www.lithos-graphics.com.