I read an enlightening discussion on HN a few months back about "hardness". I was under the impression professional scientists/researchers do not use the generic terms "hard" or "hardness". Outside of marketing, I presume they would employ concise measurements like ductility, elastic stiffness, plasticity, strain, strength, toughness, viscoelasticity, and viscosity.
EDIT: nvm found the answer via Google
https://www.quora.com/Why-is-the-diamond-hard-yet-brittle