Petrographical comparison of old and new control stones for the accelerated polishing test

A petrographical examination of the old and new control stones used for the accelerated polishing test shows that whilst the old control stone is an alkali granite, the new control stone is a dolerite. In spite of this difference the two stones give the same ‘polished stone value’, and it is suggested that this is because they have a similar mean hardness as calculated from their observed mineralogical composition.