Strong, dense, and resilient quartzite, basalt, and various other particularly strong rocks. (f=20)
II. Very strong granite, quartz porphyry, siliceous schist, relatively strong quartzite, strong sandstone, and limestone (f=15)
Strong and dense granite, strong sandstone and limestone, quartz veins, strong conglomerate, and strong iron ore (f=10)
IIIa Strong and sturdy sandstone, limestone, marble, dolomite, pyrite, and unstable granite. (f=8)

IV Strong and average sandstone and iron ore (f=6)

IVa is relatively strong sandy shale and shale sandstone. (f=5)
V Medium strong and sturdy mudstone shale, weak sandstone and limestone, soft gravel. (f=4)
Various unstable shales and dense marls with moderate strength (f=3)
People have realized in long-term practice that some rocks are not easy to break, while others are difficult to break. Rocks that are difficult to break are generally also difficult to drill and blast, so their hardness is relatively high, which can be summarized as relatively strong. Therefore, people use the concept of rock strength to represent the ease of rock fragmentation.