Large Single Nodule of Yenisofça / Eskişehir Opal, Turkey

This beauty is a Yenisofça opal nodule from the Eskişehir area with some quite complex structures - it is not unknown for these to be confused with thundereggs. Especially if, like this one, they contain a distinct 'core'. However, these are found in sedimentary rocks rather than volcanic, and common opal replaces an earlier more clay-based nodule (sepiolite). There are references to both transparent opal and chalcedony from here, and I think this one is leaning towards a chalcedony core.

11.5cm