Here, we have developed a new approach for the complete recovery of single-phase rs-ZnO synthesized at high-pressure -high-temperature conditions from commercially available w-ZnO microcrystalline powders. We demonstrated that the sample environment (capsule of pyrolytic graphite-like BN) and the compression geometry (3-axial compression in a Walker-type multianvil (MA) high-pressure apparatus ) play an important role in phase stability of the resulting nanostructured rs-ZnO. To avoid the reverse transition, 3-axial decompression is an effective tool, as is the nature of the w-ZnO precursor and the presence of isostructural compounds. The recovered bulk ingot of single phase rs-ZnO exhibits the highest hardness ever achieved for ZnO materials and the highest thermal stability -for the metastable rock-salt phase of ZnO.