| Rock Characteristics | Approximate Range of Unconfined Compressive Strength | |||
| Hardness | Consistency | Field Identification | PSI | MPa |
| R0 | extremely soft rock | Indented by thumbnail | 28 - 100 | 0.19 - 0.69 |
| R1 | very soft rock | Crumbles under firm blows with point of geologic pick, can be peeled by a pocket knife | 100 - 1000 | 0.69 - 6.9 |
| R2 | soft rock | Can be peeled by a pocket knife with difficulty, shallow indentations made by firm blow of geological pick | 1000 - 4000 | 6.9 - 27.6 |
| R3 | average rock | Cannot be scraped or peeled with a pocket knife, specimen can be fractured with single firm blow of hammer end of geological pick | 4000 - 8000 | 27.6 - 55.2 |
| R4 | hard rock | Specimen required more than one blow with hammer end of pick to fracture it | 8000 - 16,000 | 55.2 - 110.3 |
| R5 | very hard rock | Specimen required many blows of hammer end of geological pick to fracture it | 16,000 - 32,000 | 110.3 - 220.6 |
| R6 | extremely hard rock | Specimen can only be chipped with geological pick | >32,000 | >220.6 |