Indentation fracture is widely used in the ceramics and hard-metals industries for the measurement of material fracture toughness, Particular advantages unique to this technique are the relative ease of performing tests, the small quantity of material required, the low cost and ready availability of equipment, and the simplicity of sample preparation. We have applied this technique successfully to the measurement of fracture toughness of geological materials and measurements have been made on a number of samples, These measurements have been correlated with experimental results from particle breakage tests under impact conditions. The very small volume of material involved in the indentation test makes the technique useful as a microstructural probe, It can be used to measure the relative fracture resistance of different portions of a mineral composite, This is done by measuring the total extent of the surface expression of cracking produced by a micro-indenter using image analysis procedures, Measurements in a number of mineralogical materials showed that the amount of fracturing in the vicinity of a grain boundary is more extensive than in the bulk of the grain, and that in some materials, fracture along the grain boundary dominates over propagation into the adjacent grain. These effects could have significant implications for mineral liberation.