The Mohs scale indicates the hardness of minerals by noting how scratch resistant they are. The test was developed in 1812 by the German mineralogist Friedrich Mohs. Diamond, the hardest known substance, is a 10 on the Mohs scale; talc, one of the softest known substances, is a 1.
The test is simple enough for a layperson to conduct at home. The tester lays a flat piece of the test material against a table and tries to gouge a scratch in it with a substance of known hardness. If a scratch appears, the test material is softer than the material used to test it.