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.
Beryl (7½ to 8)
Spodumene (6½ to 7 - Kunzite & Hiddenite)
Andradite garnet (Demantoid & Melanite)
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.
Note how the absolute hardness value moves exponentially, when compared to the Mohs' scale value.