From Ohio History Central
In 1822, a German mineralogist named Friedrich Mohs devised a hardness scale using scratching as his testing force. Mohs used naturally occurring minerals as his standards. His scale runs from 1, the softest point on the scale, to 10, the hardest. It should be understood, however, that the Mohs scale does not represent an exact mathematical relationship. Diamonds are not ten times as hard as talc, neither is corundum twice as hard as topaz. In fact, there is a more accurate scale that has been put together using a precise laboratory instrument. On that scale diamonds have a hardness of 1000 (the scale runs from 0 to 1000), corundum has a hardness of only 250, and topaz has a hardness of 160. Still, mineralogists generally have found the Mohs scale useful since its acceptance.
Common materials other than the Mohs standard minerals have been inserted into the Mohs scale. Indeed, an imaginative worker might wish to add additional materials to suit her or his own needs, and with attention and care this could be done.
Because of these known differences in hardness, the Mohs Scale becomes one of a number of tools that can be used in the process of identifying unknown mineral specimens. A mineral or other material with a higher hardness number can scratch anything with an equal or lower number. Thus, a copper penny can scratch calcite, gypsum and talc, while a fingernail can scratch only gypsum and talc. A piece of quartz can scratch fluorite but it cannot scratch topaz.
The Mohs Hardness Scale With Other Materials
|5||apatite||steel knife blade or window glass|