What is hardness?
The hardness of a mineral is its ability to resist scratching.
Where did the hardness scale originate?
Friedrich Mohs, a German mineralogist, developed a hardness scale over 100 years ago. The hardest mineral known, diamond, was assigned the number 10.
How does the hardness scale work?
The Mohs Hardness Scale ranks the order of hardness of minerals and some common objects. For example, your fingernail can scratch the minerals talc and gypsum, with a hardness of 2 or lower. A copper penny can scratch calcite, gypsum, and talc.
A common misunderstanding of how to identify a diamond is that it will scratch glass. While this is true, other minerals can scratch glass too as long as they have a hardness > 6.