Hardness is the ability of a smooth surface to resist scratching by other surfaces. Variation in hardness has to do with microscopic chemical structures within a mineral. This means you can learn things about a mineral from scratching it that could otherwise only be known by looking through a high powered microscope. Because many minerals may look the same to the naked eye, differences in hardness can provide major assistance in identification. Scratch tests can narrow down the possibilities when trying to identify minerals of similar shape, color, and lustre.