Hardness

Schmidt Rock Hardness Classification

Grade	Classification	Field Identification	UCS [MPa]	PLT [MPa]	Examples
R6	Extremely strong	Specimen can only be chipped with the geological hammer	> 250	> 10	Fresh basalt, chert, diabase, gneiss, granite, quartzite
R5	Very strong	Specimen requires many blows of geological hammer to fracture	100–250	4–10	Amphibolite, sandstone, basalt, gabbro, gneiss, granodiorite, peridotite, rhyolite, tiff
R4	Strong	Specimen requires more than one blow of geological hammer to fracture	50–100	2–4	Limestone, marble, sandstone, schist
R3	Moderately strong	Cannot be scraped or peeled with a pocket knife; specimen can be fractured with a single firm blow of a geological hammer	25–50	1–2	Concrete, phyllite, schist, siltstone
R2	Weak	Can be peeled with a pocket knife with difficulty; shallow indentations made by a firm blow with point of geological hammer	5–25		Chalk, claystone, potash, marl, siltstone, shale, rock salt
R1	Very weak	Crumbles under firm blows of geological hammer; can be peeled with a pocket knife	1–5		Highly weathered or altered rock, shale
R0	Extremely weak	Indented by thumbnail	< 1		Stiff fault gouge