Young's Durability of Gemstones Scale - The Complete Paper

Young’s Durability Scale: A Probabilistic Elemental Model for Gemstone Durability

By Kennon Young, GG, CBJT, ASA-MGA
Vermont Gemological Laboratory, 2025

Abstract

“The degree of hardness, however, cannot be the sole determinant of a mineral’s resistance to wear or destruction. Many minerals, though quite hard, are very brittle and may fracture or cleave easily, while others, though not particularly hard, exhibit remarkable toughness and resilience. Thus, hardness and durability must not be confused.”
—Friedrich Mohs [1]

Durability remains one of the most misunderstood and oversimplified properties in gemology, frequently conflated with Mohs hardness. In reality, gemstone durability—the resistance to scratching, breaking, chipping, thermal or mechanical shock, and structural failure—is a complex, multifactorial, and inherently probabilistic set of attributes. Due to the highly variable nature of crystallographic structures, inclusion patterns, and chemical compositions across and within species, durability cannot be defined through a single deterministic metric.

This paper introduces Young’s Durability Model, a comprehensive, weighted framework for evaluating gemstone durability through a multidimensional lens. The model identifies and quantifies three primary factors influencing breakage potential: hardness, fracture toughness, and inclusions. Each parameter is assigned a relative weight proportional to its contribution to failure probability across gem species [2][3].

The result is a novel 2D visual matrix and ranking index encompassing gemstones for which sufficient scientific data exist. This visualization enables gemologists, designers, and trade professionals to interpret a gemstone’s comprehensive durability at a glance.

This model corrects widespread misconceptions—such as the misapplication of hardness as a standalone durability measure or the overestimation of certain visually “tough” species—by grounding each ranking in atomic-scale structural chemistry and stress scenarios [4][5].

It is important to emphasize that this model does not attempt to describe absolute performance under every possible condition. Rather, it reflects the statistically probable behavior of each species under common usage scenarios. Exceptional examples—such as flawless emeralds or unusually stable opals—are excluded, as they are outliers compared to typical stones used in the jewelry industry.

The scale is not linear or uniform: adjacent rankings may represent different magnitudes of durability depending on the nature and weighting of their risk factors.

This work builds upon and extends prior scholarship published in Gems & Gemology by GIA and by the author (Young, Fall 2018, Volume 54, Issue 3), advancing gemstone science through a more comprehensive and probabilistic framework for applied durability.

Methodology

The Three Elements of Durability

This model introduces a three-part framework: hardness, fracture toughness, and inclusions. When appropriately weighted, these traits help determine the relative durability of a gemstone.

Hardness (35% weight)
Measures a gemstone’s resistance to surface scratching, traditionally assessed using the Mohs scale. This parameter is normalized onto a 1–10 scale [6].

Fracture Toughness (45% weight)
Refers to the gemstone’s ability to resist crack propagation and chipping. Measured in erg/cm² and logarithmically scaled, this trait is critical for predicting real-world breakage or cleavage susceptibility [2][7].

Inclusions (20% weight)
Evaluates how typical internal inclusions influence structural integrity. Using a qualitative scale from –5 (significant weakening) to +5 (significant strengthening), this component captures how features like feathers, needles, or interlocking aggregates affect gemstone strength [3][8][9].

The final score is scaled non-linearly—much like Mohs’ approach—where gaps between values vary to reflect substantial material performance differences. Half-point values (e.g., 8.5) allow for greater resolution across species [1][6].

Parameters and Critical Assumptions

To maintain analytical consistency and practical relevance, the following parameters and assumptions were defined:

Gem-Quality Material Only
All measurements, rankings, and qualitative assessments are based on facet-grade, gem-quality specimens. Stones are assumed to be suitable for jewelry use, free of macroscopic damage, and representative of materials encountered in the commercial market [3].

Crystallographic Form Reflecting Jewelry Norms
 Durability metrics reflect the typical crystallographic or structural form in which each gemstone is used:

-Nephrite jade, a fibrous polycrystalline aggregate, is evaluated in this form due to              its ubiquity in carvings and cabochons [10].

-Turquoise is assessed as a microcrystalline aggregate.

-Corundum, spinel, and other singly refractive minerals are evaluated in their single            crystal state unless otherwise noted [11].

This ensures the model captures behavior in real-world conditions, not idealized lab specimens.

Mohs Hardness Interpolated with Real-World Weighting
 While Mohs hardness is ordinal—ranking minerals by scratch resistance without implying equal intervals—this model treats it as a continuous, interpolated variable. The large leap in absolute hardness between corundum (Mohs 9) and diamond (Mohs 10) is factored in using published Vickers and Knoop data [6][12]. This nuanced view enables hardness to contribute meaningfully within the tri-element framework, with its imprecision balanced by the heavier weighting given to fracture toughness.

Inclusions Are Scored Qualitatively
 The inclusion score (+5 to –5) is based on:
Frequency (GIA "Type 1," "Type 2," "Type 3") [8]

-Visibility and size

-Distribution

-Interaction with cleavage and crystal symmetry

Some stones—e.g., nephrite, jadeite, or amber—may benefit structurally from intergrowths, earning positive values. Others—like emerald or tanzanite—suffer reduced durability due to inclusions and cleavage, resulting in negative scores [9]. Scores were then linearly normalized to match the 1–10 scale used for hardness and toughness.

Scale is Relative and Non-Linear
The final durability score is mapped to an ordinal, categorical 1–10 scale—like Mohs. This allows meaningful distinction even when differences in input data are marginal. For example, diamond and sapphire are both highly durable but are scored differently to reflect nuanced differences in performance under pressure [4][13].

Results

Using the weighted model, 20 gemstones were analyzed and scored:

Conclusion

By synthesizing hardness, toughness, and inclusion stability, Young’s Durability Scale offers a comprehensive, probabilistic, and practical tool for jewelers, gemologists, designers, collectors, auction houses, and consumers.

This tri-element model captures the complexity of gemstone wear and breakage resistance more accurately than any singular scale. Its non-linear, weighted system reflects how these materials behave under real-world conditions and provides actionable insights into gemstone selection, usage, and care.

Bibliography

[1] Mohs, F. (1870). On the Degree of Hardness of Minerals. Historic Mineralogical Society Archives.
(Translated from the original German manuscript. Source often cited in Mohs-related literature.)

[2] Tromans, D., & Meech, J. A. (2002). Fracture toughness and surface energies of minerals: Theoretical estimates for oxides, sulfides, silicates, and halides. Minerals Engineering, 15(12), 1027–1038. https://doi.org/10.1016/S0892-6875(02)00280-6

[3] GIA Gemological Institute of America. (n.d.). Gem Inclusion Types. Retrieved from https://www.gia.edu/gia-news-research-Inclusions

[4] GIA Gemological Institute of America. (2022). Colored Stone Grading Lab Manual, 2022 edition.

[5] Read, P. G. (2005). Gemmology (3rd ed.). Butterworth-Heinemann. https://www.sciencedirect.com/book/9780750658579/gemmology

[6] GIA Gemological Institute of America. (n.d.). Gemstone Hardness: Understanding Durability. Retrieved from https://www.gia.edu/gem-hardness-durability

[7] Sun, Y., Liu, X., Zhang, L., & Tan, R. (2021). Quantitative toughness modeling of common jewelry minerals. Materials Performance and Characterization, 10(3), 345–356. https://doi.org/10.1520/MPC20200115

[8] Koivula, J. I., & Kammerling, R. C. (1986). Gem News: Update on inclusion classification. Gems & Gemology, 22(1), 57–59. https://www.gia.edu/gems-gemology/spring-1986-gemnews

[9] Muhlmeister, S., & Fryer, C. (1997). Gemstone enhancement and durability. Gems & Gemology, 33(4), 250–264. https://www.gia.edu/gems-gemology/winter-1997-gemstone-enhancement

[10] The Journal of Gemmology. (2019). Aggregate structure and toughness in nephrite and jadeite. Journal of Gemmology, 36(3), 203–215. https://gem-a.com/journal-of-gemmology

[11] Nassau, K. (2001). Gemstone Enhancement: History, Science and State of the Art (2nd ed.). Butterworth-Heinemann. https://www.elsevier.com/books/gemstone-enhancement/nassau/978-0-7506-5330-4

[12] Lawn, B. R. (1993). Fracture of Brittle Solids (2nd ed.). Cambridge University Press. https://doi.org/10.1017/CBO9780511623127

[13] Klein, C., & Dutrow, B. (2007). Manual of Mineral Science (23rd ed.). John Wiley & Sons. https://www.wiley.com/en-us/Manual+of+Mineral+Science%2C+23rd+Edition-p-9780471721574

[14] Nassau, K. (2001). Gems Made by Man (2nd ed.). Butterworth-Heinemann. https://www.elsevier.com/books/gems-made-by-man/nassau/978-0-7506-5332-8

[15] Gem-A Teaching Notes. (2023). Durability Factors of Pearls and Opals. Gemmological Association of Great Britain. https://gem-a.com