Experimental works have been performed on five types of limestone, two types of marble and two types of granite representing sedimentary, metamorphic and igneous rocks respectively. Rate of penetration (ROP) of diamond core drill with different thrust load and rotary speeds have been obtained. Drilling Specific energy (SE) has been determined in all types of rocks under investigation at different applied loads and rotary speeds. The results were obtained for rate of penetration (ROP) to show the variation in specific energy with the different rocks. A new dimensionless index UCS/SE (Uniaxial Compressive Strength divided by specific energy) was calculated and the rates of penetration against UCS/SE for all rocks were plotted. The interpretation of these relationships clears that at lower thrust loads and higher rotary speeds the three groups of rocks were classified distinctly as three categories. Whereas, at higher thrust loads and higher rotary speeds were clearly classified as two categories, one for sedimentary only and the other for metamorphic and igneous together. From these results with other information obtained by analysis of drill cuttings and the results that have been obtained from relationships, it can be possible to identify the rock type being drilled.