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Rock PSI

    Rock
Formation
  Approximate
MOH'S Hardness*
Soft Rock   Talc   1
0-6k psi   Shale   Less than 3
    Clay   0.7 - 2.8
    Mudstone   1.5 - 2.8
    Gypsum   2
Medium Rock   Calcite   3
7k-12k psi   Limestone   2.2 - 3.3
    Marble   2.4 - 3.2
    Slate   4 - 5
    Fluorite   4
    Apatite   5
    Sandstone   3 - 6
Medium Hard Rock   Dolomite   5.2 - 6.7
13k-25k psi   Graywacke   5.6 - 6.8
    Feldspar   6
    Granite   6 - 7
    Schist   6 - 7
    Gneiss   6 - 7
Hard Rock   Quartz   7
26k-45k psi   Quartzite   6 - 7
    Diorite   7 - 7.5
    Basalt   6.8 - 7.8
    Taconite   7 - 7.8
Very Hard Rock   Dyke   8
46k+ psi   Topaz   8
    Corundum   9
    Lava   9