Mohs scale of mineral hardness
Published descriptions of these spheres being harder than steel are definitively false in terms of Mohs scale of mineral hardness. Steel can vary from 4 to 8 on the Mohs scale, depending on the type of alloy, heat treatment, and whether it is case-hardened or not. An examination of several Klerksdorp spheres found none to be harder than 5.0 on the Mohs scale. For comparison, common glass has a Mohs hardness of 5.5.