In mineralogy, hardness commonly refers to a material's ability to penetrate softer materials. An object made of a hard material will scratch an object made of a softer material. Scratch hardness is usually measured on the Mohs scale of mineral hardness. Pure diamond is the hardest known substance so it will scratch any other material. Diamond, being the hardest material, is the substance used to cut diamond. Higher-grade diamonds are used to cut lower-grade diamonds.

From the physical point of view, hardness is not a precisely defined characteristic. It includes a whole series of properties based upon the cohesion of minerals and depending to a great extent upon cleavage. If the cleavage plane is vertical to the plane under inspection, the hardness is lowest in the direction parallel to the cleavage rifts. Perpendicular to them it is highest. The cohesion between individual grains in polycrystalline minerals plays an important part in the hardness determination. Considerable hardness differences are found in polymorphic minerals, such as graphite and diamond, the size of their atoms and ions and the bonding between them plays an important part. Minerals with smaller atoms and ions tend to be harder; and those with larger atoms and ions tend to be softer. Hardness is one of the most helpful diagnostic features for obtaining quick identification of minerals in the field.

Friedrich Mohs devised a scale in 1812 to assign the relative hardness of minerals. It has ten grades represented by a set of standard minerals producing a white streak. They are arranged in order of increasing hardness. The difference in hardness between neighbouring minerals in the Mohs' scale varies from small differences among the lowest grades to larger differences among the last minerals. It is an unusual scale in that each unit is rated relative to the others; there is no direct linear progression of hardness. For example, a diamond (rating 10) is actually 4 times harder than corundum (9) but 1600 times harder than talc (1).

More precise methods of determining the hardness based on the resistance of minerals to abrasion (Rosiwal), and to scratching with a diamond point, are applied mainly in testing metals and in the ceramics industry.

As with the Rosiwal scale, the Knoop scale values of hardness are absolute. They depend on the depth of the signs engraved on the mineral with a standard force using a diamond point.