The Mohs Hardness scale was devised by a German geologist and mineralogist, Friedrich Mohs, in 1812.
This hardness scale is based on the ability of one natural sample of mineral to scratch another mineral visibly. The mineral that scratches the other is the harder of the two minerals. That is, Mohs hardness is the resistance to scratch by a harder mineral.
In the chart above, it shows the hardness of minerals in comparison to common objects.
The Mohs scale is strictly a relative scale and not an absolute scale. In terms of absolute hardness, diamond is actually 4 times harder than corundum.
Hardness test can be practical in the field when you want to know whether your specimen mineral is harder than the standard one you are using.
I always carry with me a Quartz crystal (hardness 7) in the field. This is useful for me to differentiate between rough Jadeite Jade (hardness of 7) and rough Nephrite Jade (hardness of 6). If the specimen can be scratched then it is not likely to be a Jadeite Jade.
Warning: This is a destructive test. For jade carvings, this test is useful when you want to differentiate between Jadeite and Nephrite. Choose an inconspicuous place and scratch it. If it leaves a dent then it is not likely that the jade sculpture you are holding is of Jadeite material.
Photo & source credit: lapigems.com, www.nps.gov, estatediamondjewelry.com