The nucleation point around which the spherulite or eventually the thunderegg has evolved, can be of various shape and texture.
Phenocryst – crystal (as quartz, oligoclase, biotite) already present in the flowing lava. Phenocrysts mostly are rather small (< 2 mm). Probably that is why phenocrysts are not that often to be seen as nucleation point. The chances exposing it through cutting a thunderegg are not large.
Some localities have produced eggs that have multiple nucleationpoints; but somehow one of them has been dominating in the eggs growth. Socalled snowflakes, which can occur multiple, seem to be formed around phenocrysts.
Xenolith – a piece of ‘foreign’ material caught in the lava flow, can have been the point of nucleation. Some localities have many eggs with xenoliths, most of the places where thundereggs are found xenoliths are rare or seem just not to be present. Xenoliths can be small, sometimes are almost as large as the thunderegg itself.
Gasbubble – In some thundereggs a smaller gasbubble is found as a nucleation point.
Unclear is if the gasbubble really has been there, caught in the lavaflow, or is the result of weathering/dissolving of a xenolith or phenocryst.