Mohs scale is a scale for determining the relative hardness of a mineral according to its resistance to scratching by one of the minerals in the table below. The scale was devised by the German mineralogist Friedrich Mohs (1773–1839).
The degree of hardness is an aid in identifying the minerals. Diamonds are harder than quartz and will therefore scratch quartz, quartz will scratch calcite, calcite will scratch gypsum, and so on. To help identify minerals, geologists have assigned numbers to the hardness of several minerals. In this hardness scale, the softer minerals are assigned a low number and the harder minerals a higher number.
In the field, an easy way of estimating the hardness of a mineral is by trying to scratch it with common objects such as a fingernail with a hardness of 2.5, or a pocketknife, hardness 5.5. Glass has a hardness of slightly less than 6 and will scratch most minerals.
To test a mineral for hardness, try to scratch it with one of these common objects. Minerals with a hardness of 6 or more will easily scratch a piece of glass. A sample such as calcite is too soft to scratch glass but is hard enough to scratch a fingernail. Therefore it has a hardness between 6 and 2.5.