Mg is a low-cost, earth-abundant, and biocompatible plasmonic metal. Fine tuning of its optical response, required for successful light-harvesting applications, can be achieved by controlling Mg nanoparticle size and shape. Mg's hexagonal close packed crystal structure leads to the formation of a variety of unique shapes in colloidal synthesis, ranging from single crystalline hexagonal platelets to twinned rods. Yet, shape control in colloidal Mg nanoparticle synthesis is challenging due to complex nucleation and growth kinetics. Here, we present an approach to manipulate Mg nanoparticle shape by one-pot synthesis followed by colloidal etching with polycyclic aromatic hydrocarbons. We demonstrate how tips and edges in faceted Mg nanoparticles can be preferentially etched to produce quasi-spherical nanoparticles with smooth surfaces. The developed approach provides an essential shape control tool in colloidal Mg synthesis potentially applicable to other oxidising metals.