The Mohs Hardness Scale is by far the most commonly used hardness scale.
It rates minerals on a scale of 1 to 10, with 1 being the softest and 10 being the hardest.
At a 1 on the scale is talc, the softest mineral. At a 5 on the scale is apatite, with a medium value of hardness. At a 10 is diamond, the hardest mineral.
A way to discover where a mineral lands on the Mohs scale is to try a scratch test. In this test, you take two minerals and streak one of minerals on the other one to see if it leaves a scratch. If the mineral does leave a scratch, it is harder than the mineral being scratched. If the other mineral instead leaves a scratch on the mineral being used to scratch, the mineral being used to scratch is harder.
Here's an example: If you try to scratch a diamond with some talc, the talc will come away with a rather large scratch as diamond is much harder. Conversely, if you scratch talc with a diamond, the talc will still come away with a scratch because it is much softer than the diamond. You can use trial and error to find the hardness of a mineral if you are not sure.
