In an industrial crystallizer, contact between crystals and crystal-crystallizer parts increases the rate of production of potential nuclei. In cases where the impact results in the formation of cracks on the crystal contacted, these newly formed nuclei can be attrition fragments. In an attempt to determine under which conditions attrition is an important cause of nucleation, indentation experiments were performed on ten different organic and inorganic substances and the consequent crack formations were studied. The amount of work necessary to form cracks is taken as a measure of brittleness, since this quantity makes a comparison to impact energies in a stirred vessel possible. To highlight the relevance for crystallization processes, indentation experiments were also performed in saturated solutions. For the majority of the crystalline substances under investigation the work necessary to form cracks is orders of magnitude lower than impact energies usually experienced in a crystallizer. This comparison indicates that for these substances attrition is always a source of nucleation in stirred vessels. An increase in the nucleation rate will result in a decrease of the median product size in the crystallizer. It is shown that the measure of brittleness can explain the greatly differing influence of the stirring intensity on the change in the median crystal size, when different substances are crystallized. A simple indentation test makes it possible to estimate whether nucleation by attrition can exert an important influence on the resulting particle size distribution of a crystallization process.