... diamond drilling to minimum. Hot-pressed Diamond Drill Bit Hardness and Rock Drillability Grades FormRock Drillability GradeBit Hardness ... diorite porphyry, quartz diorite, quartz biotite schist.830°~32°,33°~35°Granite, skarn diorite, garnet skarn, quartz ... plagioclase migmatites, diorite, quartz diorite, porphyritic granite.1030°~32°,33°~35°Silicified marble rocks, skarn, mixed-plagioclase gneiss ...