The postulate that the constant-volume friction angleof a granular material is unique and a function of mineral composition is verified experimentally. Granular materials comprised of particles ranging from minerals to metals are tested in a ring shear apparatus. Test samples are subjected to large shear displacements until a constant lower bound friction angleis mobilized. Possible effects of confining pressure, initial packing density, gradation, and particle shape on the value ofare investigated. Friction angles mobilized in drained shear at the instant of maximum contraction and in undrained shear at phase transformation and steady state are compared withvalues. The experimental results confirm a broader fundamental significance ofas a material parameter in that it is a consistent minimum drained friction angle equal to friction angles mobilized at phase transformation and steady state in undrained shear.Key words: granular materials, sand, friction angles, constant volume, steady state, phase transformation state, ring shear test.