Elaboration

Graphite spheroids appear in liquid metal, not solid. As solidification progresses, the spheroid is surrounded by an austenite sheath through which carbon must diffuse to crystallize on the graphite spheroid. This phenomenon can be seen in the thermal analysis of spheroidal graphite cast iron. The "temperature vs. time" curve shows no eutectic plateau, reflecting the lack of equilibrium between the liquid and solid phases, and hence the loss of contact between liquid and graphite.

Spheroids are formed from "heterogeneous" seeds, foreign particles whose crystalline system enables graphite crystallization (which occurs in the hexagonal system).

Magnesium is not the only element that promotes the formation...