Mohs scale

The Mohs scale of mineral hardness is a qualitative, ordinal scale ranging from 1 to 10 that classifies minerals based on their scratch resistance. A harder material can visibly scratch a softer one, allowing geologists and collectors to identify minerals rapidly in the field. German geologist Friedrich Mohs introduced the scale in 1812 in his work Attempt at an Elementary Method for the Natural-Historical Determination and Recognition of Fossils, establishing a comparative method that remains widely used today.

Historical Background

The principle of assessing hardness through scratch resistance is ancient, with references found in the writings of Theophrastus and later Pliny the Elder. However, Mohs was the first to formalise the practice into a ten-step scale using common minerals as benchmarks. His choice of minerals ranged from talc, the softest, placed at 1, to diamond, then the hardest known natural substance, set at 10.
The Mohs scale is one of several ways to define hardness in materials science. Other methods, such as the Vickers or Brinell tests, provide more quantitative results by measuring indentation rather than scratch resistance. Nonetheless, the simplicity and portability of the Mohs system make it valuable for fieldwork and basic mineral identification.

Reference Minerals and Scratch Testing

Each number on the Mohs scale corresponds to a reference mineral. These minerals are widely occurring and exhibit distinct, consistent hardness values:

• 1 — Talc
• 2 — Gypsum
• 3 — Calcite
• 4 — Fluorite
• 5 — Apatite
• 6 — Orthoclase feldspar
• 7 — Quartz
• 8 — Topaz
• 9 — Corundum
• 10 — Diamond

Scratch testing involves determining the hardest mineral that the unknown sample can scratch or the softest mineral that can scratch the sample. For instance, if a mineral is scratched by apatite (5) but not by fluorite (4), its hardness lies between 4 and 5.
Scratching, for the purposes of the Mohs scale, requires producing a visible, non-elastic deformation. While softer minerals can create microscopic dislocations on harder ones, these minor marks are not considered scratches under the scale’s criteria.
The scale is ordinal rather than proportional. Corundum (9) is roughly twice as hard as topaz (8), whereas diamond (10) is about four times as hard as corundum. This non-linear spacing limits the scale’s precision but does not detract from its practical field use.

Hardness Variability and Composite Materials

Some substances assigned values on the Mohs scale are not pure minerals. Examples include glass (typically around 5.5) and steel (ranging widely depending on alloy and treatment). Hardness determination becomes misleading when dealing with rocks, which are mixtures rather than single minerals. Granite, for example, cannot be given a single hardness value because its components—such as quartz, feldspar, and mica—each possess different hardnesses. A quartz-rich granite behaves differently from one rich in softer minerals.

Applications

Despite its limitations, the Mohs scale remains a practical tool in several fields:
Field GeologyIts ease of use allows geologists to identify minerals quickly using scratch kits or simple tools like steel points, pennies, or shards of glass.
Grinding and MillingKnowledge of a material’s hardness helps determine the appropriate milling techniques and grinding media for mineral processing and industrial reduction.
Electronics ManufacturingThe scale is used in assessing the scratch resistance of materials in flat-panel displays, protective glass for mobile devices, and other consumer electronics components.

Comparison with Other Hardness Scales

More quantitative hardness tests, such as the Vickers scale, measure resistance to indentation rather than scratching. These methods provide absolute hardness values and are better suited to engineering applications. Nonetheless, correlations can be drawn between Mohs values and Vickers numbers, allowing the two systems to be compared when necessary.
The Mohs scale endures as a simple, intuitive, and historically significant method of assessing mineral hardness. Although not suitable for precise industrial evaluation, it continues to serve geologists, educators, and hobbyists as a fundamental tool for understanding and identifying minerals.