Exploring Mohs Hardness: A Primer

Mohs Hardness is a fundamental concept in geology, yet its practicality for rockhounds is limited as it alone does not effectively aid in stone identification. However, once a stone is identified, it becomes a valuable tool for lapidaries to predict its workability. Moreover, for jewelry buyers, knowing a stone's durability for a ring is crucial, with a Mohs rating over 5.5 generally indicating suitability. In my view, Mohs hardness is indispensable in determining a stone's suitability for jewelry, but it lacks utility for amateur rockhounds on its own. It should be utilized alongside other geological techniques like cleavage and luster for comprehensive stone identification. Without grasping and applying these fundamental geological principles, Mohs hardness proves inadequate in identifying stones, given the vast array of mineral and rock types.

The Historical Evolution of Hardness Measurement

The History of Hardness:

The journey of understanding hardness dates back centuries. In 1812, German geologist Friedrich Mohs introduced the Mohs hardness scale, revolutionizing mineral identification. Mohs' scale ranked minerals from 1 (softest) to 10 (hardest) based on their ability to scratch one another. This qualitative ordinal scale provided a simple and accessible method for geologists and enthusiasts to assess mineral hardness without sophisticated equipment.

Understanding Mohs Hardness and Its Applications

Mohs Hardness: A Basic Field Guide:

Mohs hardness scale remains a cornerstone of mineral identification, particularly for enthusiasts exploring the great outdoors. Its simplicity and ease of use make it indispensable for fieldwork, where laboratory equipment might not be readily available. As a qualitative ordinal scale, Mohs hardness provides a ranking rather than precise measurements of hardness. This makes it ideal for quick assessments but less suitable for precise scientific analysis.
Diving into Hardness Measurement Methods
Methods of Hardness Measurement:

In contrast to Mohs hardness, true hardness refers to the material's resistance to deformation or abrasion under specific conditions. Unlike the qualitative ordinal nature of Mohs scale, true hardness can be quantitatively measured using various methods, including scratch tests and indentation tests. True hardness provides more precise and scientific insights into a material's properties, making it essential for detailed geological studies and industrial applications.

Methods of Hardness Measurement

The True Hardness Scale typically utilizes quantitative methods such as indentation tests or scratch tests to measure hardness. These methods yield measurements in various units depending on the specific test used. For instance:

  1. Vickers Hardness (HV): This method involves pressing a pointed indenter into the material's surface with a controlled load. Hardness is calculated based on the size of the resulting indentation, often expressed in units of pressure such as megapascals (MPa) or gigapascals (GPa).
  2. Brinell Hardness (HB): Here, a hard spherical indenter is pressed into the material's surface with a specified load and duration. The resulting indentation's diameter is measured, and hardness is calculated using force per surface area units like kilograms-force per square millimeter (kgf/mm²) or megapascals (MPa).
  3. Rockwell Hardness (HR): This method involves pressing an indenter into the material under load and measuring the depth of penetration. The difference in depth before and after the load is applied determines the Rockwell hardness value, typically expressed as a dimensionless number.

The Vickers hardness test is widely employed across North America and globally due to its versatility and precision. It can accurately measure the hardness of various materials like metals, ceramics, and composites, making it valuable for materials research, quality control, and failure analysis in different industries. Its ability to measure thin samples and small areas further enhances its utility.

Meanwhile, the Brinell hardness test remains popular, particularly in industries dealing with large or rough samples such as foundries and metalworking facilities. Its simplicity and reliability in measuring the hardness of materials with heterogeneous microstructures contribute to its continued global use.
Similarly, the Rockwell hardness test enjoys widespread adoption across various industries for its versatility, ease of use, and ability to provide reliable hardness measurements for a wide range of materials including metals, plastics, and composites.

Mineral Mohs Hardness
(Ordinal Scale)
Mohs to Vickers Ratio True Hardness
(Vickers, HV)
(Nominal Scale)
Talc 1 1:2.5 1 - 5 HV
Gypsum 2 1:3.5 5 - 6 HV
Calcite 3 1:107 90 - 130 HV
Fluorite 4 1:151 140 - 170 HV
Apatite 5 1:495 400 - 600 HV
Orthoclase Feldspar 6 1:694 550 - 850 HV
Quartz 7 1:1143 800 - 1500 HV
Topaz 8 1:1492 1000 - 2000 HV
9 1:2091 1700 - 2500 HV
Diamond 10 1:8990 8000 - 10000 HV

Note: Vickers hardness values provided are approximate and serve as a general guide. They may vary depending on specific material characteristics and testing conditions. Mohs hardness is given for comparison, and true hardness measurements are based on available data and equivalents.

Note: Numbers in the "Mohs to Vickers Ratio" column have been rounded to the median value for ease of viewing.

Factors Influencing Hardness

Crystalline Structures and Hardness:

Crystalline structure plays a significant role in determining the hardness of a material. The arrangement of atoms or ions within a crystal lattice affects how the material responds to external forces, such as indentation or scratching.
In crystalline materials, atoms or ions are arranged in a regular, repeating pattern called a crystal lattice. The bonding forces between atoms or ions within the lattice determine the material's overall strength and hardness. Materials with strong, tightly bonded crystal structures tend to be harder because they are more resistant to deformation and scratching.

For example, diamond is renowned for its exceptional hardness because of its strong covalent bonds between carbon atoms arranged in a tetrahedral crystal lattice. This structure makes diamond extremely resistant to deformation and scratching, resulting in its status as the hardest known natural material.
In contrast, materials with weaker or less tightly bonded crystal structures tend to be softer and less resistant to deformation. For instance, graphite, another form of carbon, has a layered crystal structure held together by weak van der Waals forces. As a result, graphite is much softer than diamond and can be easily scratched or marked.

Therefore, the crystalline structure of a material is a crucial factor in determining its hardness, as it influences the strength and stability of the material's atomic arrangement.

Exploring Toughness: Understanding Why Softer Stones Defy Expectations

The apparent contradiction where softer stones like nephrite jade resist abrasion more than harder stones like tourmaline can be attributed to several factors beyond just hardness.

Firstly, hardness and abrasion resistance are related but distinct properties. Hardness refers to a mineral's resistance to scratching or indentation, whereas abrasion resistance involves its ability to withstand wear and tear over time. Jade's remarkable resistance to abrasion despite its relatively low hardness is due to its unique internal structure. Jade consists of tightly interwoven fibers that create a dense and compact matrix. This structure disperses abrasive forces more evenly, reducing localized wear and preserving the stone's surface integrity.

On the other hand, tourmaline may possess a higher hardness on the Mohs scale, but its internal structure might not offer the same level of resistance to abrasion. Tourmaline's crystalline structure may contain cleavage planes or weak spots that make it more susceptible to wear and chipping, despite its hardness.

Nephrite jade, serpentine, bowenite, and mica stand out among phyllosilicates for their exceptional resistance to abrasion compared to other stone varieties. While not all phyllosilicates defy expectations by exhibiting greater toughness than their Mohs number suggests, they represent a notable group that does.

Exploring the Interplay Between Hardness and Luster

Luster and Its Relationship with Hardness:

Luster is defined as the interaction of light with a material's surface
The hardness of any substance significantly influences its luster, or shininess. The harder a substance is, the brighter it shines. In the world of gemstones, a higher ranking on the Mohs scale correlates with a more brilliant luster. For instance, topaz, being one of the harder silicates, exhibits a noticeably brighter shine after being faceted compared to quartz, despite both being completely clear.

This principle of shininess extends to all materials, including metals. For jewelers, the more work-hardened (tempered) a piece is, the brighter it's polish will be. metals with a higher degree of hardness often have a more reflective surface and a brighter luster compared to softer metals. Similarly, when selecting apples at the store, the freshest and juiciest ones with pressure upon their wax spray coating will gleam brighter. As apples shrink and wither without internal pressure, their luster dulls.

Soft stones share similarities with wood in that, no matter how much they are polished, they will never shine as brightly as harder stones higher up on the Mohs scale. Just as sanding wood beyond 1200 grit yields no additional gloss, soft stones require the application of oil to achieve a glossy sheen. Many novice lapidaries overlook this necessity for soft stones. Mineral oil is often used for this purpose and is effective on materials like soapstone, marble, and jade, enhancing their luster and vitality.