ABSTRACTSIt has been established that on occasion prehistoric man heat‐treated siliceous stones in order to improve their flaking qualities. In this work we have examined samples of heat‐treated chert by means of X‐ray diffraction and have established that, in most cases, heat treatment causes an X‐ray diffraction line broadening which can be interpreted as a decrease in range of crystalline order of up to 40%, in the most extreme case. The implications of this to an understanding of the fundamental mechanisms of fracture and as a tool for archaeological studies are di