“…In the case of a rock's internal characteristics, factors such as isotropy, homogeneity, brittleness, hardness, and granularity have frequently been considered pertinent, all of which ultimately relate to the mineralogy and microstructure of particular rock types (Goodman, 1944;Callahan, 1979;Whittaker, 1994;Andrefsky, 1998). External characteristics that may be relevant include the size, shape, and regularity of the material to be knapped, as well as whether cortex is present or absent (Ashton and McNabb, 1994;Jennings et al, 2010;Smallwood, 2010;Eren et al, 2011). The presence of cortex is potentially important since it has been shown that rock types possessing cortex may in fact behave in terms of some properties (specifically rebound hardness) in two distinct ways, with cortical material acting as one distinct rock type, while the uncortical surface of the same rock can act more similarly to other rock types entirely (Eren et al, 2014).…”