The Article considers the aspect concerning the prediction of one of indices of rock mass structure,i.e.a dimension of structural blocks of fractured stony and semistony rocks – the index which is required for transition from strength (cohesion) of rocks in a specimen to cohesion in rock mass in evaluation of open-pit slope stability. It is shown also the determination of reference value of the dimension of structural blocks by core from geological holes with the aid of RQD (Rock Quality Designation) at the stage of prospecting of one of ore deposits intended for open-pit mining.
