Mohs scale

A - Z Gemstone Menu

A Comparison of Gemstone Hardness

The most common method of describing gemstone hardness is by the Mohs' scale of hardness. The Mohs' scale was devised in 1822 by Friedrich Mohs, a German mineralogist (1773-1839).

The scale compares different minerals in order of relative hardness where 10 is the hardest material and 1 is the softest. Diamond is the hardest at position 10 and Talc is the softest at position 1.

A mineral is able to scratch other minerals at a lower position on the scale, but is unable to scratch those at a higher position on the scale.

In practice, hardness pencils are manufactured to carry out this procedure. A hardness test can separate similar looking materials and help identify gem materials. In gem mining areas this is a very quick and useful test, especially in remote locations.

The Mohs' scale does not show the absolute hardness value of minerals, see below.

Mohs' Scale of Hardness

(with particular reference to gemstone materials)

10
Diamond
9
Corundum
8
Topaz
7
Quartz
6
Feldspar (Orthoclase)
5
Apatite
4
Fluorite
3
Calcite
2
Gypsum
1
Talc

The Table Showing Additional Gemstones

10
Diamond
9
Corundum
Chrysoberyl
8
Topaz
Spinel

Beryl (7½ to 8)
Zircon
Almandine garnet

Spessartine garnet
Pyrope garnet
Grossular garnet (Hessonite, Tsavorie & Hydrogrossular)
7
Tourmaline
Quartz
Jadeite jade
Iolite (7 to 7½)

Tanzanite
Spodumene (6½ to 7 - Kunzite & Hiddenite)
Peridot
Nephrite jade
Andradite garnet (Demantoid & Melanite)

6
Turquoise
Opal
Moonstone (6 to 6½)
Feldspar (Orthoclase)
5
Apatite
Lapis Lasuli
4
Fluorite
3
Calcite
2
Gypsum
1
Talc

Absolute Hardness

The Mohs' scale does not show the actual difference in hardness between each mineral (gemstone), this is shown by an absolute hardness scale.

The absolute hardness of a mineral is a measurement of the amount of force needed to scratch or indent it, using a diamond probe. There are various testing methods, the example below shows the Mohs' scale compared to the absolute hardness values from a sclerometer. The instrument measures scratch resistance by applying pressure to a moving diamond point on the surface of the mineral under test.

Comparison of the Mohs' Scale & the Absolute Hardness Scale.

Mohs' scale
Absolute hardness scale
10
1500
9
400
8
200
7
100
6
72
5
48
4
21
3
9
2
2
1
1

Note how the absolute hardness value moves exponentially, when compared to the Mohs' scale value.