The Mohs scale of mineral hardness is a simple yet effective system for assessing the scratch resistance of various materials. Developed in 1812 by German mineralogist Friedrich Mohs, it is a qualitative, ordinal scale that ranges from 1 to 10. The scale ranks materials based purely on their relative ability to visibly scratch one another, a method that has proven durable in both scientific and commercial applications.
Defining Mineral Hardness
The Mohs scale measures a material’s resistance to scratching (abrasion). The underlying principle is straightforward: a harder material will scratch a softer material, while the softer material will not mark the harder one. This relationship allows for the placement of any substance relative to the ten standard minerals on the scale.
The ten reference minerals, from softest to hardest, are:
- Talc (1)
- Gypsum (2)
- Calcite (3)
- Fluorite (4)
- Apatite (5)
- Orthoclase (6)
- Quartz (7)
- Topaz (8)
- Corundum (9)
- Diamond (10)
This system is not linear, meaning the difference in absolute hardness between steps is not uniform. For example, the jump in hardness between Corundum (9) and Diamond (10) is significantly greater than the difference between Talc (1) and Gypsum (2).
The test procedure involves attempting to scratch a sample of unknown hardness with a mineral of known hardness, or vice versa, to determine where the unknown sample falls on the scale. If a material is scratched by Apatite (5) but not by Fluorite (4), its hardness is then reported as being between 4 and 5.
Identification and Assessment in Earth Sciences
The Mohs scale is a foundational tool for field geologists and mineralogists. Geologists often use common, readily available items with a known Mohs hardness to perform quick scratch tests outside of a laboratory setting. A fingernail has a hardness of about 2.5, a copper penny is around 3.5, and a steel file is typically near 6.5.
By testing an unknown mineral against these common objects, a geologist can quickly narrow down its identity based on its approximate scratch resistance. If a sample can be scratched by a steel file but not by a copper penny, its hardness is established between 3.5 and 6.5, providing a significant clue for classification.
In gemology, the scale is used to assess the durability and wear resistance of gemstones. Gemstones with a higher Mohs hardness, like diamond (10) or sapphire (9), are resistant to the abrasion encountered in daily wear, making them suitable for rings and other frequently worn pieces. Softer gems, such as opal (typically 5.5 to 6.5), require protective settings due to their vulnerability to scratching from common materials like household dust, which often contains hard Quartz particles (7).
Industrial and Commercial Applications
Beyond mineral identification, the Mohs scale informs decisions in manufacturing. Manufacturers use the scale for quality control and material selection across various industries, including ceramics, glass, and metals. It helps in choosing materials that will resist abrasive wear in their intended application, such as ensuring a display screen or ceramic tile can withstand normal use.
The Mohs scale is particularly useful in the selection and application of abrasive materials for cutting and grinding. Abrasives must be harder than the material they are intended to cut or polish. Industrial processes frequently employ Corundum (9) or synthetic materials like silicon carbide to grind or shape metals and other tough substances. Diamond, with its Mohs hardness of 10, is used in drill bits and cutting tools for the hardest materials.
The scale helps evaluate the durability of surface coatings and finishes used on flooring, countertops, and electronic devices. Manufacturers test protective coatings to ensure they provide adequate scratch resistance against common contaminants, from the finish on a wood floor to the hardened glass used on a smartphone screen.