Hardness Is Not Toughness — Harder Materials Are Often More Brittle

Introduction

In the gold, silver, and gemstone trade, a belief has persisted for a very long time:

A diamond rates 10 on the Mohs scale, so it cannot be scratched, chipped, or damaged.

This view is only partly true.

The confusion arises from equating two entirely different concepts in materials science: hardness and mechanical durability, or fracture toughness (Toughness/Fracture Toughness).

Over nearly 30 years of work in gemstone grading, we have regularly recorded used diamonds showing:

• facet scratches (Facet Scratch);
• facet junction wear (Facet Junction Wear);
• girdle chips (Girdle Chip);
• facet edge damage (Facet Edge Damage);
• crystal fractures (Fracture);
• and even breakage along a cleavage plane (Cleavage).

These phenomena are entirely consistent with the physical nature of diamond and are described in both gemology textbooks and modern materials science.

1. Mohs hardness reflects only scratch resistance

The Mohs scale, devised in 1812 by the German mineralogist Friedrich Mohs, compares the scratch resistance of minerals.

Diamond is placed at Mohs 10, the natural material with the highest scratch resistance.

This means only that:

• a diamond can scratch almost any other material;
• very few materials can leave a scratch on a diamond's surface.

It does not mean a diamond cannot crack, chip, or break.

Mohs hardness says nothing about resistance to impact.

2. Vickers hardness reflects resistance to deformation

In modern materials science, hardness is often assessed with the Vickers Hardness (HV) scale.

Diamond's hardness is about 70–150 GPa, or roughly 7,000–10,000 HV, the highest of all natural materials.

This figure, however, reflects resistance to plastic deformation when pressed by a standard diamond indenter.

It does not reflect:

• resistance to cracking;
• resistance to breakage;
• resistance to impact.

A material with extremely high hardness can therefore still break.

3. Fracture toughness determines resistance to cracking and breaking

In fracture mechanics, the most important property is fracture toughness (KIC).

It expresses a material's ability to resist the propagation of a crack.

Diamond's fracture toughness is only about 3–7 MPa·√m, considerably lower than that of many engineering metals.

For example:

• alloy steel: about 50–150 MPa·√m;
• titanium: about 50–100 MPa·√m.

This explains why a diamond is very hard to scratch yet cracks more easily than steel when struck in the right direction.

4. The crystal structure creates both the hardness and the weakness of diamond

Diamond crystallizes in the cubic (isometric) crystal system.

Its carbon atoms are joined by very strong covalent bonds in a tetrahedral arrangement. This is the source of diamond's exceptional hardness.

Within the crystal, however, lie perfect cleavage planes of the {111} family.

Along these planes the interatomic bonding perpendicular to the plane is weaker than in other directions. If an impact force travels along this direction, a crack spreads very fast and the diamond can split in two.

For hundreds of years, diamond cutting has exploited this very property to cleave rough diamonds.

5. Stress concentration at facet junctions

During cutting and polishing, the facets meet to form very sharp lines called facet junctions.

Mechanically, this is where stress concentration occurs.

When a ring strikes:

• the edge of a stone countertop;
• a granite tile floor;
• a stair handrail;
• a car door frame;
• a hard metal surface,

the entire impact force converges on an extremely small area.

The local stress at the facet junctions can be many times the average stress across the whole stone.

Once it exceeds the failure limit, the following occur:

• facet edge damage;
• a girdle chip;
• facet junction wear;
• a crystal fracture.

6. Why is the girdle the most chip-prone spot?

The girdle is the thinnest cross-section of a diamond.

If it is cut extremely thin, it becomes almost the stone's weak point.

A single hard enough blow can produce a girdle chip.

That is why international laboratories always assess girdle thickness as an important criterion.

7. Scratches and facet junction wear reduce optical performance

A diamond possesses:

• a very high refractive index;
• strong dispersion;
• nearly perfect total internal reflection.

These properties reach their best effect only when:

• the facet surfaces are perfectly polished;
• the facet edges are sharp;
• the cut angles keep the correct optical geometry.

When scratches, abrasion, nicks, or chips appear, the path of light is altered.

As a result:

• more light leaks out;
• reflection decreases;
• dispersion decreases;
• the diamond looks less bright, shows less fire, and may appear darker or less white under some viewing conditions.

This is especially noticeable in diamonds of high cut quality.

8. Significance for grading work

Mechanical damage, even very small, is often impossible to see with the naked eye.

With, however:

• a standard 10× loupe;
• a gemological microscope;
• fiber-optic illumination;
• specialized optical observation equipment,

an experienced grader can readily detect:

• scratches;
• abrasion;
• nicks;
• girdle chips;
• cleavage cracks;
• feathers arising after an impact.

Accurately assessing these signs matters greatly for:

• verifying the condition of the diamond;
• comparing against a prior grading record;
• detecting signs of replacement or substitution;
• determining the cause of damage;
• supporting the resolution of disputes between customers and traders.

Conclusion

Hardness of 10 Mohs is a diamond's most striking property, but it does not mean immunity to every form of mechanical damage.

From a materials-science perspective, the very crystal lattice that gives diamond its superior hardness also leaves it with relatively low fracture toughness and perfect {111} cleavage planes, so the stone can still chip, crack, or break under a strong enough blow in the right direction.

Understanding the difference between hardness, fracture toughness, and impact resistance not only helps consumers wear and care for diamond jewelry correctly, but also affirms the role of in-depth grading in assessing a diamond's true condition throughout its life of use.

For professional laboratories such as SJC Chợ Lớn, detecting and accurately assessing micro-damage on diamonds is not only a matter of expertise but also an important scientific basis for protecting customers' interests, improving market transparency, and building trust in independent grading.