... can be used as required.3. Hot-pressed Diamond Drill Bit Hardness and Rock Drillability Grades FormRock ... 26° ~28°Silicified siltstone, talc through amphibolite, olive marble, white marble, quartz diorite porphyry, black schist, diopside marble rock, marble ... , 22° ~25° , 26° ~28Black hornblende plagioclase gneiss, white plagioclase gneiss, quartz, dolomite marble, black marble, mica, dolomite, ...