Limitations of the Mohs Hardness Scale
Ordinal vs. Quantitative Measurement
The Mohs scale, developed in 1812 by German mineralogist Friedrich Mohs, ranks minerals from 1 (talc) to 10 (diamond) based on scratch resistance. Each mineral can scratch those below it but not those above. While useful for field identification of minerals, this ordinal scale has significant limitations for engineering applications.
The Mohs scale is non-linear and non-quantitative. The difference between Mohs 9 (corundum) and 10 (diamond) represents a far greater absolute hardness gap than between 1 and 2. Gold ranks 2.5–3 on this scale, similar to a fingernail (2.5) or calcite (3), but this tells us nothing about how much harder 14K gold is compared to 24K gold.
Why Mohs Doesn't Predict Jewelry Performance
Jewelry durability depends on multiple mechanical properties beyond scratch resistance: indentation hardness (resistance to permanent deformation under load), yield strength (stress required to cause permanent deformation), tensile strength (resistance to pulling forces), fatigue resistance (ability to withstand repeated stress cycles), and abrasion resistance (resistance to material removal through friction).
The Mohs scale addresses only scratch resistance and provides no information about these other critical properties. Two materials with identical Mohs hardness can have vastly different performance in jewelry applications.
Scratch Resistance vs. Structural Integrity
A material can be scratch-resistant yet brittle (like glass or ceramic), or soft yet tough (like pure gold). Jewelry requires a balance: sufficient hardness to resist surface damage, adequate toughness to absorb impacts without fracturing, and appropriate ductility to allow fabrication and repair.
The Mohs scale cannot distinguish between these properties, making it inadequate for jewelry material selection or quality assessment.
Quantitative Hardness Testing Methods
Vickers Hardness (HV): Industry Standard
The Vickers test uses a diamond pyramid indenter with a 136° angle between opposite faces. A known load (typically 1–120 kgf for metals) is applied for 10–15 seconds, creating a square indentation. The diagonal lengths are measured optically, and hardness is calculated as load divided by surface area of the indentation.
Vickers hardness (HV) provides quantitative, reproducible measurements across a wide hardness range. It's the preferred method for precious metal alloys because the small indentation size allows testing of jewelry components without significant damage, the pyramid geometry works well for both soft and hard materials, and results correlate well with other mechanical properties like yield strength.
Brinell Hardness (HB): Bulk Material Testing
The Brinell test uses a hardened steel or tungsten carbide ball (typically 10mm diameter) pressed into the material under high load (500–3000 kgf). The diameter of the resulting circular indentation is measured and used to calculate hardness.
Brinell testing is less common for jewelry due to large indentation size (unsuitable for small components), lower precision for very soft or very hard materials, and potential for ball deformation when testing hard alloys. However, it's useful for testing bulk alloy ingots before fabrication.
Knoop Hardness (HK): Thin Sections and Coatings
The Knoop test uses an elongated diamond pyramid indenter, creating a narrow, elongated indentation. This geometry allows testing of thin coatings, plating, or small features without edge effects or substrate influence.
Knoop hardness is valuable for evaluating gold plating thickness and hardness, testing small jewelry components, and assessing surface-hardened layers. The elongated indentation requires less depth than Vickers, making it ideal for thin sections.
Rockwell Hardness: Less Common for Precious Metals
Rockwell testing measures indentation depth rather than area, using either a diamond cone or hardened steel ball. While fast and simple, Rockwell scales (A, B, C, etc.) are optimized for ferrous metals and industrial alloys. The method is rarely used for gold jewelry due to limited sensitivity in the soft-to-moderate hardness range and indentation size unsuitable for small components.
Hardness Values Across Gold Purities
24K Gold: 25–30 HV
Pure annealed gold exhibits Vickers hardness of 25–30 HV. This extremely low value reflects gold's FCC crystal structure and ease of dislocation movement. Work hardening through mechanical deformation can increase this to 50–60 HV, but the effect is limited and reversed by annealing.
22K Gold: 50–80 HV
22K gold (91.7% Au, 8.3% alloying metals) shows modest hardness increase through solid solution strengthening. The exact value depends on alloy composition—copper-rich formulations reach the higher end of this range, while silver-rich alloys remain softer.
18K Gold: 125–165 HV
18K gold (75% Au, 25% alloying metals) represents a significant hardness jump. Typical values: yellow 18K (balanced Cu-Ag) reaches 125–140 HV, rose 18K (copper-rich) achieves 135–150 HV, and white 18K (nickel-based) attains 160–180 HV, while palladium-white 18K measures 150–165 HV.
14K Gold: 135–200 HV
14K gold (58.5% Au, 41.5% alloying metals) provides maximum hardness among commonly used jewelry alloys. Yellow 14K typically measures 140–160 HV, rose 14K reaches 145–165 HV, and white 14K (nickel-based) achieves 170–200 HV.
10K Gold: 140–220 HV
10K gold (41.7% Au, 58.3% alloying metals) can achieve very high hardness, particularly in nickel-white formulations. However, the high alloy content raises concerns about tarnish, color consistency, and whether the material retains gold's desirable characteristics.
Hardness vs. Other Mechanical Properties
Tensile and Yield Strength
Hardness correlates with tensile strength (resistance to pulling forces) and yield strength (stress causing permanent deformation). Empirical relationships allow estimation of these properties from hardness measurements. For gold alloys, tensile strength (MPa) ≈ 3.5 × HV, though this varies with alloy composition and processing.
This correlation makes hardness testing valuable for quality control—a single non-destructive test provides insight into multiple mechanical properties.
Elastic Modulus and Stiffness
Elastic modulus (Young's modulus) measures stiffness—resistance to elastic (reversible) deformation. Gold's modulus (~80 GPa) is lower than steel (~200 GPa) but higher than silver (~70 GPa). Alloying has modest effect on modulus compared to its dramatic effect on hardness.
Stiffness determines how much jewelry flexes under load. Lower modulus means more flexibility, which can be desirable for comfort but problematic for structural components like prongs.
Fatigue Resistance Under Cyclic Loading
Fatigue failure occurs when repeated stress cycles cause crack initiation and propagation, even at stresses below yield strength. Jewelry experiences fatigue in clasps (repeated opening/closing), chains (flexing during wear), and rings (thermal cycling and mechanical stress).
Hardness alone doesn't predict fatigue resistance—microstructure, defects, and stress concentrations play critical roles. However, harder alloys generally show improved fatigue life in jewelry applications.
Abrasion and Wear Resistance
Abrasive wear occurs when hard particles or surfaces remove material through micro-cutting or plowing. Wear resistance generally increases with hardness, though other factors (lubrication, surface finish, contact geometry) also matter.
For jewelry, wear resistance determines how quickly surfaces develop scratches, how fast ring shanks thin, and how long prongs maintain their shape. Higher hardness translates directly to longer-lasting surface finish.
Practical Implications for Jewelry Longevity
Ring Shank Wear Patterns
Ring shanks experience continuous abrasion against adjacent fingers, work surfaces, and environmental particles. Wear is most severe on the palm side, where contact frequency is highest. A 24K gold ring might show measurable thinning within months, while 14K gold maintains thickness for years under equivalent conditions.
Prong Durability and Stone Retention
Prongs must resist both wear (gradual material loss) and deformation (bending under impact). Minimum hardness of 100–120 HV is recommended for reliable prong settings. This explains why 24K gold (25–30 HV) is unsuitable for prong settings, while 14K and 18K alloys provide adequate security.
Surface Finish Retention Over Time
Polished surfaces develop micro-scratches through contact with harder materials. The rate of finish degradation correlates inversely with hardness. A 14K ring might maintain mirror polish for 6–12 months of daily wear, while 18K requires refinishing after 3–6 months, and 24K shows visible scratching within days.
Hardness Testing Comparison
| Test Method | Principle | Load Range | Best For | Gold Alloy Range |
|---|---|---|---|---|
| Mohs | Scratch resistance | Qualitative | Mineral ID | 2.5–3 (all purities) |
| Vickers (HV) | Diamond pyramid indentation | 1–120 kgf | Jewelry alloys | 25–200 HV |
| Brinell (HB) | Steel ball indentation | 500–3000 kgf | Bulk metals | 50–180 HB |
| Knoop (HK) | Elongated diamond indentation | 1–1000 gf | Thin coatings | 30–220 HK |
| Rockwell | Indentation depth | Varies | Industrial metals | Rarely used |
Frequently Asked Questions
What is the Mohs hardness of gold?
Pure gold ranks 2.5–3 on the Mohs scale, similar to a fingernail. However, this ordinal scale doesn't reflect the quantitative hardness differences between 14K, 18K, and 24K gold.
Is Vickers hardness more accurate than Mohs for jewelry?
Yes. Vickers hardness (HV) provides quantitative, reproducible measurements that correlate with wear resistance, making it far more relevant for jewelry performance assessment.
Why is 14K gold harder than 18K if both are gold?
14K gold contains more alloying metals (~41.5% vs. 25%), which increase hardness through solid solution strengthening. Vickers hardness reflects this: 14K averages 150–180 HV vs. 125–140 HV for 18K.
Does harder gold mean more durable jewelry?
Generally, yes—but hardness is only one factor. Toughness, fatigue resistance, and design also affect durability. Extremely hard alloys can be brittle.
Can gold be hardened after manufacturing?
Gold alloys can be work-hardened through mechanical deformation (hammering, rolling), but they cannot be heat-treated like steel. Annealing reverses work hardening.
Internal Links
For context on how hardness differences affect long-term wear, see our materials science comparison of gold hardness explained across purities.
Learn about alloy composition effects in How Gold Alloys Affect Strength, Color, and Wear.
Compare mechanical properties in Why Pure Gold Is Rarely Used in Everyday Jewelry.
References
This article references ASTM E384 (Vickers and Knoop hardness testing), materials testing handbooks, metallurgy textbooks on mechanical properties, and jewelry industry technical standards.