Recently developed microform measurement techniques have reduced the measurement uncertainties in the geometry of Rockwell diamond indenters. It is now possible to establish standard-grade Rockwell diamond indenters characterized by high geometric uniformity, high hardness performance uniformity, interchangeability and reproducibility. By using the standard indenters under different national standard machines and a standardized testing cycle, a world-wide unified Rockwell hardness scale could be established with metrological traceability, stability and reproducibility. Geometric measurements and hardness tests in five laboratories have shown that tightly controlled indenter geometry can significantly improve the consistency of Rockwell C hardness (HRC) measurements. These results support the feasibility of establishing a world-wide unified HRC scale with an expanded uncertainty of approximately +/- 0,2 HRC and without significant bias with respect to an ideal scale.