Mohs Scale of Mineral Hardness

In mineralogy, "hardness" is defined as the ability of one mineral to scratch another. You use this concept in every day life when using a nail file or sandpaper - the nail file should have a hardness greater than your fingernail, and the sandpaper (covered with tiny fragments of a particular mineral) should be harder than whatever is being sanded.

Friedrich Mohs was the first to codify mineral hardness in about 1820. Although there are other, more quantitative hardness tests available, the Mohs scale has remained popular because it is easy to use.

Mohs Scale of Hardness (with common items included)

To use this scale, you take a piece of each mineral/substance, and see if it scratches the other. 

The materials with a * have variable hardness so use common sense when testing minerals with them. For example, a steel knife blade should always scratch calcite, but some types of steel may not scratch apatite.