Sapphire is a precious gemstone and a variety of corundum, which is a crystalline form of aluminum oxide. Beyond its aesthetic appeal, sapphire is renowned for its exceptional durability. This property makes it highly valued for fine jewelry and demanding industrial applications. To understand its resilience, one must examine how this material ranks on the scale used to measure mineral scratch resistance.
The Mohs Scale of Mineral Hardness
The Mohs scale, developed by German mineralogist Friedrich Mohs in 1812, formalizes the method for assessing a mineral’s resistance to scratching. This system is a qualitative, ordinal scale that ranges from 1 to 10. It characterizes scratch resistance by observing whether a harder material can visibly scratch a softer one. The fundamental principle is simple: any material can scratch all others below it and will be scratched by any material above it.
The scale is anchored by ten reference minerals, starting with Talc (1) and ending with Diamond (10). The full list of reference minerals is:
- Talc (1)
- Gypsum (2)
- Calcite (3)
- Fluorite (4)
- Apatite (5)
- Orthoclase (6)
- Quartz (7)
- Topaz (8)
- Corundum (9)
- Diamond (10)
The Mohs scale is not linear, meaning the jump in absolute hardness between consecutive numbers is not uniform. For instance, the difference between 9 and 10 is far greater than the difference between 1 and 2. This comparative system provides a straightforward and practical method for quick identification and classification of minerals. The scale only measures resistance to surface scratching, which is distinct from a material’s toughness or resistance to fracturing upon impact.
Sapphire’s Exact Ranking
Sapphire holds a position near the top of the hardness ranking, registering a score of 9 on the Mohs scale. Both sapphire and its red counterpart, ruby, are composed of the mineral corundum, which is aluminum oxide. This high ranking is a direct result of corundum’s dense crystalline structure, where atoms are tightly packed and held together by strong chemical bonds.
The position of the material on the scale provides context for its exceptional durability. Sapphire is substantially harder than Topaz, the reference mineral immediately below it at a ranking of 8. In terms of absolute hardness, the corundum crystal at 9 is approximately twice as hard as topaz.
The only naturally occurring mineral ranked higher is Diamond, which holds the maximum value of 10. Despite being only one number apart, the absolute hardness difference between sapphire and diamond is dramatic. Diamond is exponentially harder than corundum, making it the only substance capable of easily scratching sapphire. This distinction emphasizes the non-linear nature of the scale.
Practical Implications of Hardness
A Mohs hardness of 9 translates directly into a high degree of scratch resistance, benefiting both consumers and industry. For jewelry, this hardness means sapphires are exceptionally well-suited for daily wear. This high rank ensures the gemstone will maintain its polish and sharp facets over many years of use.
The material’s extreme hardness provides protection against the most common source of scratching: airborne dust and grit. These particles frequently contain Quartz, which has a Mohs ranking of 7 and can scratch many common materials. Because sapphire is significantly harder than quartz, it is impervious to scratching from this ubiquitous abrasive.
This resistance to wear is why synthetic sapphire, often marketed as sapphire glass, is used in specialized high-tech products. It is employed for watch crystals and protective covers over smartphone camera lenses, where scratch resistance is necessary. The material’s ability to withstand abrasion also makes it ideal for industrial windows, optical components, and barcode scanners that endure constant contact.