Tunnel boring machine (TBM) penetration tests entail incrementally increasing TBM thrust from full stop to maximum speed and recording the penetration rate either at set thrust levels or at set times. TBM penetration test data can be analysed by plotting the penetration rate (distance/revolution) against the net cutter thrust (force per cutter) over the full range of thrust levels in the test, called the penetration-thrust graph. This research shows that the transition from excavation dominated by grinding to excavation dominated by chipping can be observed in penetration-thrust graphs. Correlating penetration test data to the geological and geomechanical characteristics of rock masses through which a penetration test is conducted provides the ability to reveal the efficiency of the chipping process in response to changing geological conditions. By analysing penetration test data from projects in the Swiss Alps, and published data from Singapore, this research shows that the strength of the rock is an important control on how much net cutter thrust is required to transition from grinding to chipping. It also shows that the geological characteristics of a rock will determine how efficiently chipping occurs once it has begun. In particular, geological characteristics that lead to efficient fracture propagation, such as fabric and mica content, will lead to efficient chipping. These findings will enable a better correlation between TBM performance and geological conditions for use in TBM design, as a basis for contractual payments where penetration rate dominates the excavation cycle and in further academic investigations into the TBM excavation process.