Hardness

Definition

A measure of the ease with which a smooth surface of a mineral can be scratched, or of its resistance to abrasion.

In 1822 the Austrian mineralogist Friedrich Mohs devised a scale based on one mineral's ability to scratch another. He placed 10 minerals in order from softest to hardest, giving a relative hardness value of 1 to the softest mineral, and 10 to the hardest. Each mineral in the scale scratches the one below it (the lower number) but not the one above it (the higher number). This became known as Mohs' scale of hardness, and is still one of the best practical methods of estimating a mineral's hardness. Mohs designated ten minerals for his scale from hardest (10) to softest (1) (listed with objects of equivalent hardness):

10	Diamond
9	Corundum
8	Topaz
7	Quartz	(porcelain - 7)
6	Orthoclase	(steel file - 6.5)
5	Apatite	(knife blade or nail - 5, window glass - 5.5)
4	Fluorite
3	Calcite
2	Gypsum	(fingernail - 2)
1	Talc

diamond	corundum	topaz	quartz	orthoclase
apatite	fluorite	calcite	gypsum	talc

Test

Using moderate pressure, drag a sharp edge over the smooth surface of a mineral. If the surface of the mineral is scratched then it is softer than the material used to scratch it, if not then it is harder.

Terminology

Soft - can be scratched by a fingernail, Mohs' 1-2;
Medium - can be scratched by a knife or nail, Mohs' 3-5;
Hard - cannot be scratched by a knife but can scratch glass, Mohs' 6-9;
Diamond is the hardest known mineral, Mohs' 10.

Notes: It must be noted that Mohs' scale is arbitrary and non-linear, i.e. the steps between relative hardness values are not necessarily equal. Rather, it is a method of gauging the relative hardness of a mineral. If a mineral cannot be scratched by a knife blade but can be scratched by quartz then its hardness is between 5 and 7 (stated as 5-7) on Mohs scale. A relative hardness value of 6.5 means that the mineral could scratch orthoclase (feldspar) but not quartz.