The hardness of a material is probably the most important characteristic to consider when deciding on what type of mill to choose. This page is about hardness of kinds of minerals. 

The hardness of a material is probably the most important characteristic to consider when deciding on what type of mill to choose. Trying to grind a material which is too hard, such as sand in most types of beater mill, will result, either in costly damage to the mill or an expensive maintenance requirement. Most types of readily available hammer mills for agricultural grinding are not suitable for grinding most types of minerals.

Hardness of minerals is expressed on Mohs’ scale - a numerical index ranging from 1 for talc (the softest mineral) to 10 for diamond (the hardest known material). Table below shows Mohs’ scale of hardness.

The following form is the simple description about the different stone hardness:

In general, the harder the material, the more specializing and expensive the type of mill used has to be. In addition if a particular mill can be used over a range of hardness scales, the harder the material the lower the throughput for a given size requirement. Another characteristic of a material to be aware of is brittleness, which is the degree to which a material will easily break. Most minerals are brittle, as opposed to metals which are ductile, although some to a greater degree than others. Brittleness does not equate with hardness as brittle materials can be hard or not particularly hard. Materials which are not brittle to some degree, metals or soft plastics for example, cannot easily be milled.