  Though extensively studied, hardness, defined as the resistance of a material
to deformation, still remains a challenging issue for a formal theoretical
description due to its inherent mechanical complexity. The widely applied
Teter's empirical correlation between hardness and shear modulus has been
considered to be not always valid for a large variety of materials. Here,
inspired by the classical work on Pugh's modulus ratio, we develop a
theoretical model which establishes a robust correlation between hardness and
elasticity for a wide class of materials, including bulk metallic glasses, with
results in very good agreement with experiment. The simplified form of our
model also provides an unambiguous theoretical evidence for Teter's empirical
correlation.
