The Mohs hardness scale is a standardized measure of the relative hardness of various stones and minerals. It was generated in 1812 by a German geologist and mineralogist, Friedrich Mohs, after whom the scale is named. The scale ranges from 1 to 10, with 1 being the softest and 10 being the hardest. Quartz, the most common mineral, has a hardness of 7.
Though this scale is imprecise, it is very useful as a means of identifying and classifying minerals and stones. It can also act as a guide when people are considering the care and storage of their stones; that is, one will want to be careful not to carry a hard stone, such as topaz, in the same place as a soft stone, such as amber, as the harder stone can scratch, crack, or otherwise damage the softer one.