If the nucleation points are distributed along the wall of a cavity, the typical wall-lining banding develops. Depending on the density of the initial nucleation points on the wall, the banding pattern will look more jagged like in so-called "fortification agates" (left column in Fig.) or will follow the outline of the cavity more uniformly (right column in Fig.). I have marked the different developmental stages of the spherulites with different colors to emphasize the geometry of the resulting banding.
|Fairburn Agate, credit: Tom "Captain Tenneal" Shearer|