The Mohs Hardness Scale is used to help identify minerals. A mineral's hardness is a measure of a mineral's resistance to scratching. This is measured by scratching it against another substance of known hardness on the Mohs Hardness Scale. This simple graphic outlines the index minerals and common objects used to determine a mineral's hardness.
To perform a hardness test, the harder mineral (higher number) will scratch all minerals with a lower number. If it just leaves a streak on the other sample it is equal in hardness.