There is a site for Thundereggs (Lithophysae) that contain Opal nearby Housaka in Fukushima Prefecture, Japan. Typically, the Opal is white and opaque. A gravitational caused banding is visible. Rarely opalecent precious Opal can be found at this site.
Hideharu Yamada is offering a lot of information about japanese agate and others on his website www.lithos-graphics.com.