In material systems with competing phase stabilities, simple growth techniques for nanocrystal synthesis are generally restricted to producing the thermodynamically lowest energy crystal structure. Alumina is one industrially and scientifically relevant example where the excess enthalpy of high specific surface area structures (i.e., nanocrystals) forces synthesized nanocrystals to adopt crystal structures that deviate from those observed in large crystals and have very different properties. Prior room temperature thermodynamic calculations determine that alumina nanocrystals smaller than 12 nm are either γ or amorphous, and numerous experimental works confirm the difficulty of producing α-alumina nanocrystals smaller than ∼15 nm. In this work, a direct and simple, high-energy processing route was developed in order to reduce as-grown ∼50 nm α-alumina starting crystallites down to sub-10 nm crystallites; below the room temperature thermodynamic crossover size limit. The nanocrystals were subsequently washed to remove WC-Co contamination on the nanocrystal surfaces incurred during the high-energy process. The smallest crystallite size of high-energy processed powder was 8.7 nm. High-resolution TEM establishes that the individual crystallites have different crystallographic orientations with nominal sphericity revealing a truly nanocrystalline powder. The developed processing route provides an industrial and scalable procedure that provides a new avenue for nanocrystalline α-alumina synthesis and, potentially, other metastable nanocrystals.