What does 'Harder than diamond' mean?

Syntheses of new classes of hard and superhard materials1,2,3 provide new challenges for the measurement of hardness. In the past, the criteria for such measurements were based on a careful comparison and semiquantitative appraisal of materials with respect to each other. The best example of this is the Mohs hardness scale, which categorizes materials in terms of their ability to scratch one another, with diamond at the top with a value of 10. But for materials that are purportedly harder than diamond, using such relative criteria become problematic, which in turn raises the question of what it means — in quantitative terms — for a material to be harder than diamond. This question was the subject of intense debate amongst delegates at a recent workshop on superhard materials*, and this correspondence summarizes the consensus of these discussions