The scratch method for measuring mineral hardness was devised by German mineralologist Friedrich Mohs in 1822 and although crude, the same method is used today throughout the world. A scratch on a mineral is a groove that can only be made by a harder mineral. Since a softer mineral cannot scratch a harder mineral, a relative scale was established to document the differences in hardness. Mohs scale of hardness is a rough measure of the resistance of a smooth surface to abrasion or scratching by another harder mineral.

The Mohs' scale of hardness is a misnomer because the numbers assigned to different minerals are not proportional to their scratch resistance. Rather than a scale, the numbers represent a table or an ordered list, and the table does not give a continuous range of measurements. For example, the hardest element with a 10 on the mohs hardness scale (diamond) is 4 times harder than an element that measures 9 (corundum); whereas a 4 on the scale is only about 10% harder than a 3. Since the Mohs' hardness scale is a relative scale and not an absolute scale, the numbers assigned to each element should be used for reference purposes only.

There is a wood hardness scale as well, and it addresses the wood's density and hardness, but it doesn't tell you if the wood is necessarily the best wood to use for flooring. It's also a relative measure and is used to help people understand part of the picture of the durability of the wood they are considering for their flooring.

Mohs' scale measures the hardness of minerals using the scratch method but other methods such as indentation, bending, fracture, breaking, and abrasion are also valuable measures. These other hardness measures are important because durability is often confused with hardness. For example, glass is rated harder on the Mohs' scale than rubber, but rubber is more durable than glass. Consider that a glass ball, albeit harder, would break when dropped, and a softer rubber ball would bounce.

Though the Mohs' hardness scale lists ten common minerals (talc, gypsum, calcite, fluorite, apatite, orthoclase, quartz, topaz, corundum and diamond), most people don't have these minerals lying around their house. To simplify the use of the Mohs' hardness scale, common minerals have been assigned numbers: fingernails are 2, a penny is 3, a knife blade is 5, window glass is 5.5, and a steel file is 6.5. By categorizing these common household items, the ratings on the Mohs' scale are given context, making the hardness scale more meaningful and useful.

When you buy flooring tiles, be they ceramic, porcelain, or terra cotta, you'll often see references to the Mohs' Scale of Hardness. That number doesn't give you the complete picture of the durability of the tile you are buying, but it is a start in giving you an understanding of how the flooring tiles will last and hold up to the abuse your floor receives.