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