Mohs scale

The Mohs scale is a qualitative ordinal scale used to characterize the scratch resistance of minerals.

It ranks materials from 1 to 10 based on their ability to scratch one another. In this system, a harder material will scratch a softer material. This scale is a vital tool for geologists working in the field to identify minerals. However, it is not a precise way to predict how materials will behave in industrial settings. While it shows the order of hardness, it does not measure absolute hardness.

To determine a mineral's position, scientists observe how it reacts to contact with other substances. The mechanism involves testing which material can leave a visible mark on another. Technically, a scratch is defined as creating non-elastic dislocations that are visible to the naked eye.

Sometimes, a softer material might create microscopic dislocations on a harder material. These tiny marks are permanent and can affect structural integrity. However, they are not considered true scratches for the purposes of the Mohs scale. To find a value, you identify the hardest material a specimen can scratch or the softest that can scratch it.

For example, if a material is scratched by apatite but not by fluorite, its hardness is between 4 and 5.

The scale relies on ten specific reference minerals that are widely found in rocks.

These minerals act as fixed points for comparison. Talc is the softest at level 1, followed by gypsum at 2. Calcite is 3, fluorite is 4, and apatite is 5. The middle of the scale includes orthoclase feldspar at 6 and quartz at 7. Higher values include topaz at 8 and corundum at 9. Diamond defines the top of the scale at 10, as it was the hardest known natural mineral when the scale was designed.

Humans have used methods of comparing mineral hardness since antiquity. Theophrastus mentioned these methods in his treatise, *On Stones*. Later, Pliny the Elder described similar observations in his work, *Naturalis Historia*. The modern version of this system was introduced in 1812.

It was created by Friedrich Mohs, a German geologist and mineralogist. He published his findings in his book, which established the organized scale used by scientists today. His work turned ancient observations into a formal scientific tool.

It is important to understand that the Mohs scale is an ordinal scale, not a linear one. This means the intervals between numbers are not equal in terms of actual strength.

For instance, corundum (9) is twice as hard as topaz (8) in absolute terms. However, diamond (10) is about four times as hard as corundum. This difference is clearly seen when comparing Mohs values to absolute hardness scales.

The scale shows the relative order, but the actual physical difference grows much larger at the higher numbers.

Testing hardness can be complicated when dealing with mixtures. A rock like granite is a mixture of several minerals rather than a single chemically pure solid.

Because granite contains different minerals, its hardness can be misleading. For example, topaz-rich granite contains topaz (8), quartz (7), and orthoclase (6), but also mica (2–4). Different sources might assign granite a hardness between 6 and 7. Even non-mineral substances can be assigned values, such as copper at 2.5–3 or glass at 5.5–6.

Today, the Mohs scale remains highly relevant across many different fields. In geology, it helps identify specimens during field research. In industrial milling, it helps experts choose the right grinding media for a product.

Electronic manufacturers also rely on it. They use the scale to test the resilience of components in consumer electronics. This includes checking the hardness of cover glass for LCD screens or the touch screens on mobile devices. It ensures these surfaces can withstand daily use without being easily damaged.