Various rock types may differ in their hardness.
|
Hardness
|
Rock Type
|
Uniaxial Unconfined Compressive Strength (UCS)
|
|
ultra soft
|
clay, gumbo
|
< 1,000 psi
|
|
very soft
|
chalk, claystone, salt
|
1,000 to 4,000 psi
|
|
soft
|
coal, schist, stilstone
|
4,000 to 8,000 psi
|
|
medium
|
sandstone, slate, shale
|
8,000 to 17,000 psi
|
|
hard
|
basalt, gabbro, quartzite
|
17,000 to 27,000 psi
|
|
very hard
|
gneiss, granite, marble
|
> 27,000
|
