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.
It is a hardness comparison scale and does not show absolute hardness. The scale shows minerals in order of relative hardness where 10 is the hardest and 1 the softest. 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. Diamond is the hardest at position ten and Talc is the softest at position one.

The Mohs' scale does not show the actual difference in hardness between each mineral, 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 varous 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.