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