Rocks may be considered to consist of natural material from the ground with adequate Mohr-Coulomb cohesion so that they do not behave as a soil. For intact rock, this value of cohesion might be set above 250 kPa. For them not to behave as a soil also requires a minimum particle size. This is less easy to define as the relative size of particles compared to the nature of engineering matters. Rocks also frequently contain discontinuities in the form of joints which may be open or filled.

A soil is an agglomeration of particles whereas a rock is not unless it is a sedimentary deposit that has undergone lithification and possibly further diagenesis.

The measurement of rock properties and stress described in the following sections refers to material that can be tested as intact rock or distinct joints therein.