Rock PSI

Rock
Formation
Approximate
MOH’s Hardness*

Soft RockTalc1
0-6k psiShaleLess than 3
Clay0.7 – 2.8
Mudstone1.5 – 2.8
Gypsum2

Medium RockCalcite3
7k-12k psiLimestone2.2 – 3.3
Marble2.4 – 3.2
Slate4 – 5
Fluorite4
Apatite5
Sandstone3 – 6

Medium Hard RockDolomite5.2 – 6.7
13k-25k psiGraywacke5.6 – 6.8
Feldspar6
Granite6 – 7
Schist6 – 7
Gneiss6 – 7

Hard RockQuartz7
26k-45k psiQuartzite6 – 7
Diorite7 – 7.5
Basalt6.8 – 7.8
Taconite7 – 7.8

Very Hard RockDyke8
46k+ psiTopaz8
Corundum9
Lava9