Mohs scale of mineral hardness

How the scale works

The test behind the scale is straightforward: a mineral that can scratch another is considered harder. The procedure is practical for fieldwork because it requires no complex equipment — just a set of reference specimens or common objects of known hardness. The property being tested is resistance to surface deformation by scratching, which depends mainly on the strength and nature of atomic bonding in the mineral and to some extent on structure and impurity content.

The ten reference minerals

1. 1 – Talc: very soft, can be scratched by a fingernail; often greasy to the touch.
2. 2 – Gypsum: soft but harder than talc; common in sedimentary settings.
3. 3 – Calcite: reacts with dilute acid and serves as a common calibration point.
4. 4 – Fluorite: typically colorful and used as an intermediate reference.
5. 5 – Apatite: a phosphate mineral often used to describe tooth and bone hardness comparisons.
6. 6 – Feldspar: a major rock-forming group with many varieties.
7. 7 – Quartz: very common in the crust and able to scratch glass.
8. 8 – Topaz: a gem-quality mineral higher on the scale.
9. 9 – Corundum: includes gem varieties like sapphire and ruby.
10. 10 – Diamond: conventionally the hardest natural mineral on the original scale.

Uses, limitations and modern context

The Mohs scale remains useful for quick, qualitative identification of minerals in the field and for teaching. It is simple to remember and apply: if a mineral can scratch quartz, it is harder than 7. However, the scale is not linear — the difference in resistance between adjacent steps is uneven. For industrial, engineering and scientific purposes, quantitative hardness tests (for example Vickers, Brinell or Knoop methods) give reproducible numerical values and are preferred for manufactured materials.

Practical tips and notable facts

To perform a scratch test safely, use fresh, clean surfaces and avoid damaging valuable specimens. Common objects commonly used as proxies include a fingernail, a copper coin or a piece of window glass; these are rough practical standards but not substitutes for laboratory measurement. Although the original scale places diamond at the top, research and synthetic processes have produced materials and forms with greater resistance in specific circumstances, so Mohs is best seen as a convenient field tool rather than an absolute measure of material strength.

The scale connects mineralogy, crystallography and practical geology: it illustrates how atomic bonding and crystal structure control a macroscopic property that is easy to observe without instruments. For further reference about Mohs and related concepts, see entries linked by name throughout this article such as the scale, hardness testing and the minerological profiles provided above.