Hadean Type Planetoid
Planetoidal Class Metal Dwarf. These worlds are very rare, simply because in typical pre-solar nebulae accretion processes will create rock worlds with metal cores. These worlds are typically only found about hot stars that have cleared all other rocky lighter material before the accretion process has completed.
Image from John M Dollan
As planets form and gain mass, they become differentiated due to both pressure and heat. What usually results is a world with a molten iron core, a mantle of somewhat lesser dense material that can be semi-molten, and a "light" crust of basaltic elements. This same differentiation occurs in Planetoids as they form. Often times, if a particular pre-solar nebula is metal rich, Planetoids will form that are composed almost entirely of heavy elements.
These are rare worlds, however, even in metal rich systems, primarily due to the fact that most of the heavy elements will have been gathered up by the sun. Indeed, it is exceedingly rare that such a world will ever attain a Terrestrial classification.
Hadeans may also be formed when a Planetoid with a large iron core has nearly all of its mantle and core blown off by a massive impact early in the formative history. The result is a world literally laid bare.