The Mohs Hardness Scale is one of the most crucial tools in gemology and mineralogy. It offers an indispensable method for determining the hardness of various minerals, a key factor when selecting gemstones or understanding their durability. For anyone deeply invested in the world of gemstones—be it for collecting, jewelry making, or scientific research—the Mohs hardness scale serves as a foundational metric.
What is the mohs hardness scale?
The Mohs Hardness Scale is a qualitative ordinal scale that ranks minerals based on their ability to scratch softer substances. Developed by German mineralogist Friedrich Mohs in 1812, the scale ranges from 1 (talc), the softest mineral, to 10 (diamond), the hardest naturally occurring substance known to humanity. The simplicity of the Mohs scale lies in its comparative nature: a mineral can scratch those below it on the scale but can be scratched by those above it.
The scale of hardness provided by Mohs has stood the test of time due to its practical applications in identifying and comparing minerals, particularly in fields like geology, materials science, and gemology. Despite being over two centuries old, the scale remains a cornerstone for understanding mineral hardness.
The history of gemstone and mineral hardness
The concept of measuring mineral hardness dates back long before Mohs, but it was his systematic approach that revolutionized the field. Before Mohs, ancient civilizations such as the Egyptians and Romans understood that certain materials were more resistant to wear and tear than others. For example, they knew that quartz, used in various tools and jewelry, could scratch other stones. However, there was no standardized method to quantify this property.
The advent of the Mohs hardness scale provided a universal method to classify minerals based on their ability to resist scratching, a property that correlates with their atomic structure and bonding. Hardness, as understood in this context, is a measure of the strength of the atomic bonds within a crystal structure.
Who was Friedrich Mohs?
Friedrich Mohs, born in 1773, was a German geologist and mineralogist whose work laid the groundwork for modern mineral classification. After studying under prominent geologists of his time, Mohs became deeply interested in how minerals could be classified beyond just their chemical compositions. He sought a more accessible, tactile method that could be employed even in the field.
In 1812, while working as a professor in Austria, Mohs introduced his now-famous scale. His hardness scale was revolutionary because it offered a simple, practical way to identify minerals based on their physical properties. This scale was not just a theoretical construct but a tool that could be applied by anyone with access to the ten standard minerals on the scale.
Understanding the mohs scale of hardness
The Mohs hardness scale is not a linear scale. The difference in hardness between minerals increases exponentially as you move up the scale. For example, diamond at 10 is not just harder than corundum (9) but is exponentially harder, as diamond’s atomic structure gives it unparalleled strength.
Each step on the Mohs scale corresponds to a mineral that can scratch all those below it but will be scratched by those above it. Here’s a breakdown of the Mohs scale:
- Talc (1): Can be easily scratched with a fingernail.
- Gypsum (2): Can be scratched with a fingernail.
- Calcite (3): Can be scratched with a copper coin.
- Fluorite (4): Can be scratched with a knife blade.
- Apatite (5): Can be scratched with a knife, but not as easily as fluorite.
- Orthoclase Feldspar (6): Can scratch window glass.
- Quartz (7): Can scratch steel and hard glass.
- Topaz (8): Can scratch quartz.
- Corundum (9): Can scratch topaz.
- Diamond (10): Can scratch all other substances.
Understanding this hierarchy is essential for anyone working with gemstones, as the hardness of minerals directly impacts their use in jewelry and their care. For instance, diamonds, which top the Mohs hardness scale, are incredibly resistant to scratching, making them ideal for daily wear, such as in engagement rings.
How to use the mohs hardness scale
In practice, the Mohs hardness scale is used by testing a material against the reference minerals listed on the scale. To determine where a gemstone or mineral falls on the hardness scale, it is subjected to scratching with minerals of known hardness.
For example, if a mineral can be scratched by orthoclase (6) but not by apatite (5), it falls somewhere between 5 and 6 on the Mohs hardness scale. This method is particularly useful in the field where laboratory equipment is not available, allowing geologists and gemologists to quickly and effectively identify minerals.
However, the Mohs hardness scale should not be confused with the material’s toughness or resistance to breaking and chipping. While a diamond is the hardest substance on the scale, it is also quite brittle and can be chipped or broken with a hard impact.
Mohs scale of hardness vs. other hardness measurements
While the Mohs hardness scale is widely used, it is not the only method for measuring hardness. Other scales, such as the Vickers Hardness Test, Brinell Hardness Test, and Rockwell Hardness Test, offer more precise measurements, particularly for industrial materials like metals and alloys. These scales measure hardness in terms of the depth or area of indentation under a specific load, rather than the scratch resistance used in the Mohs scale.
For instance, the Vickers hardness test involves pressing a diamond pyramid under a specific load into the material’s surface and measuring the diagonal of the resulting indentation. The Vickers hardness number (VHN) is then calculated, providing a more quantitative measure of hardness than the Mohs scale.
Despite these more precise methods, the Mohs scale remains popular in gemology due to its simplicity and the ease with which it can be applied without specialized equipment. It also holds educational value, offering an intuitive understanding of how different minerals compare in terms of hardness.
Selecting gemstones: beyond the mohs hardness scale
When choosing gemstones for jewelry, the Mohs hardness scale is an important consideration, but it should not be the only one. Hardness relates to a gemstone’s resistance to scratching, but toughness (resistance to breaking or chipping) and stability (resistance to chemicals and environmental factors) are equally important.
For example, while emeralds score between 7.5 and 8 on the Mohs scale, making them relatively hard, they are also known for their inclusions, which can make them more prone to chipping. Therefore, despite their hardness, emeralds require careful handling and protective settings in jewelry.
Sapphires and rubies, which are both varieties of corundum and score a 9 on the Mohs scale, are not only hard but also tough, making them excellent choices for rings and other jewelry that is subject to daily wear.
Thus, when selecting gemstones, it is crucial to consider the balance between hardness, toughness, and stability to ensure the longevity and durability of the piece.
The relationship between hardness and durability
Hardness is often confused with durability, but they are not the same thing. While hardness is a measure of a material’s resistance to being scratched, durability encompasses not only hardness but also toughness (resistance to breaking) and stability (resistance to heat, chemicals, and light).
For example, while diamonds are the hardest material on the Mohs scale, their durability can be compromised by their relative brittleness. This is why, despite their hardness, diamonds can be chipped or broken if struck at certain angles.
Conversely, jadeite, with a Mohs hardness of about 6.5-7, is much tougher than many harder materials, making it less likely to break or chip. Understanding these distinctions is crucial for anyone working with or purchasing gemstones, as it affects how the stone should be used and cared for.
Everyday applications of the mohs hardness scale
The Mohs hardness scale is not just a theoretical tool but has practical applications in various fields. In the jewelry industry, it is used to assess the suitability of gemstones for different types of jewelry. In construction, it helps in selecting materials for flooring, countertops, and other surfaces where scratch resistance is important.
The scale is also useful in education, providing a hands-on way for students to learn about minerals and their properties. Additionally, the Mohs scale plays a role in forensic geology, where it can help in identifying minerals found at crime scenes.
Understanding the hardness of minerals is also crucial in various industrial processes. For example, in drilling and cutting operations, knowing the hardness of the materials involved helps in selecting the appropriate tools and techniques.
Common misconceptions about the mohs hardness scale
One of the most common misconceptions about the Mohs hardness scale is that it is a measure of a material’s overall durability. As discussed, while hardness is an important factor, it does not account for toughness or stability.
Another misconception is that the Mohs scale is linear, with each number representing an equal increase in hardness. In reality, the scale is ordinal and non-linear, with significant differences in hardness between minerals, especially at the higher end of the scale. For example, while corundum (9) and diamond (10) are adjacent on the scale, diamond is significantly harder than corundum.
Lastly, some people mistakenly believe that the Mohs scale can be applied universally across all materials. However, the scale is specifically designed for minerals, and while it can provide some insights into other materials, it is not suitable for metals, plastics, or other non-mineral substances.