Vestian Type Planetoid
While planetesimals may typically be low-massed and geologically inert, there is a rare Type that exhibits ancient and extensive geological activity. Named the Vestian Type, in honor of the first known Planetoid of this division, the surfaces of these worlds are marked by ancient lava flows and even maria, and the crusts are composed largely of olivine and other such minerals. Theories for how such small worlds could exhibit evidence of ancient volcanism and differentiation abound, ranging from gravitational and tidal stressing in the past to these worlds actually being the remains of a Terrestrial world, catastrophically disrupted in some natural or even artificial disaster.
Image from John M Dollan
Whatever the case, these worlds remain rare, indicating that the processes which do lead to their formation are themselves rare and unknown.