Introduction
Diamond sits alone at the top of the Mohs hardness scale — a perfect 10. No natural material can scratch it. This fact is so widely known that it has become shorthand for indestructibility: diamonds are forever, diamonds are unbreakable, diamonds are the toughest thing on earth.
The first two claims are debatable. The third is simply wrong.
Hardness and toughness are different physical properties, and confusing them is one of the most common misconceptions in the gem world. A diamond resists scratching better than any other mineral, but it can absolutely be chipped, cracked, or split in two by a well-placed blow. Understanding the distinction matters — it affects how diamonds are cut, how they should be set, and how carefully they need to be worn.
This article explains what hardness actually measures, why toughness is a separate concern, and what both properties mean for anyone buying or wearing a diamond. For background on the crystal structure that gives diamond these properties, see Diamond Crystal Structure.
Key Points
What Hardness Means
Hardness, in gemology and mineralogy, is resistance to scratching — the ability of a material's surface to withstand being displaced by a harder object dragged across it. It is a surface property. The Mohs scale, devised by Friedrich Mohs in 1812, ranks ten reference minerals by their ability to scratch one another:
- Talc
- Gypsum
- Calcite
- Fluorite
- Apatite
- Orthoclase feldspar
- Quartz
- Topaz
- Corundum (ruby, sapphire)
- Diamond
The scale is ordinal, not linear. The gap between each step is not uniform. The jump from corundum (9) to diamond (10) is far larger than any other step on the scale. On an absolute hardness scale measured by indentation tests (Vickers or Knoop), diamond is roughly four times harder than corundum. That single-point difference on the Mohs scale conceals an enormous real-world gap.
What this means in practice: no natural material, and very few synthetic ones, can scratch a diamond. A diamond can only be scratched by another diamond (or by certain engineered materials such as boron nitride under specific conditions). This is why diamond remains the standard abrasive for cutting and polishing gemstones — including other diamonds.
What Toughness Means
Toughness is resistance to fracture — the ability of a material to absorb energy from an impact without breaking. Where hardness is about surface resistance, toughness is about structural resilience. A tough material can deform or absorb a blow without cracking. A hard but brittle material resists surface damage but shatters under the right kind of stress.
Diamond is hard but only moderately tough. On a comparative scale, jade (nephrite) is significantly tougher than diamond despite being far softer. Nephrite's interlocking fibrous crystal structure absorbs impact energy by distributing it across millions of tiny interwoven fibres. Diamond's rigid, directional bonding does the opposite — it transmits force efficiently along specific crystallographic planes, and if that force exceeds the bond strength across a cleavage plane, the crystal splits.
This is not a flaw. It is a consequence of diamond's atomic architecture. The same sp3 carbon bonding that makes diamond supremely hard also makes it cleave cleanly along defined directions. Hardness and brittleness, in diamond's case, share the same structural origin.
Cleavage — Diamond's Defined Weakness
Diamond cleaves along its four {111} octahedral planes — the same planes that form the faces of an octahedral crystal. Along these directions, the density of carbon-carbon bonds crossing the plane is lower than in other orientations. Apply enough force perpendicular to one of these planes, and the crystal separates cleanly.
Cleavage is not random fracture. It is a precise, directional property. A diamond cutter can position a blade along a cleavage plane and split a rough stone with a single controlled strike. This technique was the primary method of dividing rough diamonds for centuries before mechanical sawing became widespread. It is still used today for specific applications — removing a near-surface inclusion, for example, or dividing an awkwardly shaped rough crystal.
For diamond wearers, cleavage means vulnerability at specific points. A sharp blow to a thin edge, a pointed tip, or a knife-edge girdle can initiate cleavage along one of the four {111} planes. The result is a chip — a small, flat-faced break that follows the cleavage direction — or, in extreme cases, a complete fracture through the stone.
Directional Hardness — Harder in Some Directions Than Others
Diamond's hardness is not uniform. It varies with crystallographic direction — a property called hardness anisotropy. The stone is slightly harder when measured perpendicular to the octahedral {111} faces and slightly softer parallel to them. The variation is measurable: Knoop hardness tests show values ranging from approximately 5,700 to 10,400 kg/mm2 depending on the direction and crystal face being tested.
This anisotropy is not academic. It is the reason diamond polishing works at all. When a diamond cutter places a stone against a rotating cast-iron wheel (a scaife) charged with diamond powder, the wheel can only remove material from the stone if it attacks in a direction where the stone is relatively softer than the abrasive particles. If the cutter orients the stone so its hardest direction faces the wheel, the polishing effectively stalls. Experienced cutters learn to read the grain of each stone and adjust their approach accordingly.
This is also why certain facet junctions on a finished diamond can show slight polishing marks or drag lines under magnification — the faceter encountered a harder grain direction partway across the facet. Modern polishing techniques and equipment have reduced this issue, but it remains a factor in finish quality assessment.
Fracture vs Cleavage
Not all breaks in diamond follow cleavage planes. When a diamond breaks along a direction that does not align with {111}, the result is a conchoidal fracture — an irregular, curved break surface similar to what you see in broken glass. Conchoidal fracture in diamond is less common than cleavage because the cleavage planes are well-defined and preferential, but it occurs under high-energy impacts or when force is applied in directions between cleavage planes.
The distinction matters for assessment. A chip along a cleavage plane has a flat, step-like surface. A conchoidal fracture has a curved, shell-like surface. Both are damage, but a cleavage break is more likely to propagate if the stone is struck again in the same direction, because the exposed cleavage surface provides a ready initiation point for further splitting.
Chipping Risks in Everyday Wear
Diamonds in jewellery face real-world impacts: knocking a ring against a door frame, dropping an earring onto a tile floor, catching a pendant on a zipper. Whether these incidents cause damage depends on several factors:
Where the blow lands. Thin areas are most vulnerable. A knife-edge girdle — a girdle polished to an extremely thin edge — concentrates impact force at a single line and is the most chip-prone feature on a finished diamond. Pointed tips on marquise, pear, and heart shapes are similarly at risk. The feather-thin edge at the tip has minimal material to absorb force.
The direction of the blow. A strike aligned with a cleavage direction is far more dangerous than one that hits the stone at an oblique angle to all four {111} planes. This is partly why round brilliants, which have no exposed points and typically feature a medium to slightly thick girdle, are the most durable shape in practice.
The presence of existing inclusions. An inclusion near the girdle or a surface-reaching feather can act as a stress concentrator. Under impact, the crack may propagate from the inclusion along a cleavage plane, turning a minor flaw into a visible chip.
Setting design. The setting is the first line of defence. Prong settings that grip the girdle securely but do not create pressure points protect better than loose or poorly positioned prongs. V-prongs or bezels on pointed tips shield the most vulnerable areas. Channel and bezel settings, which enclose the girdle entirely, offer the highest level of physical protection.
Practical Guidelines
- Round brilliant — the most chip-resistant shape due to its continuous curved girdle and absence of sharp points. A medium girdle provides good protection without hiding excessive weight.
- Marquise, pear, heart — protect pointed tips with V-prongs or partial bezels. Avoid wearing these shapes during manual work.
- Emerald and Asscher — step-cut corners are less vulnerable than points, but the open table makes internal features more visible. Bevelled corners on emerald cuts already reduce chipping risk.
- Princess — the four sharp, unprotected corners are the most chip-prone feature of any standard shape. V-prongs on all four corners are essential.
- Girdle thickness — avoid extremely thin (knife-edge) girdles. A thin to medium girdle balances protection and face-up appearance.
- Remove rings during high-impact activities. Gardening, gym work, home repairs — any activity involving hard surfaces and sudden contact increases chipping risk.
- Insure significant stones. Even with careful wear, accidents happen. Insurance provides practical protection where physics cannot.
Frequently Asked Questions
Can a diamond break or chip?
Yes. Diamond is the hardest natural material (Mohs 10), meaning nothing can scratch it, but it can chip or split along its four cleavage planes. Thin edges, pointed tips, and knife-edge girdles are the most vulnerable areas. A sharp blow at the right angle can cause damage.
What is the difference between hardness and toughness?
Hardness is resistance to scratching — a surface property. Toughness is resistance to fracture — the ability to absorb impact without breaking. Diamond excels at hardness but is only moderately tough. Jade (nephrite), for example, is far softer than diamond but significantly tougher due to its interlocking fibrous structure.
Which diamond shapes are most prone to chipping?
Shapes with sharp, exposed points are most vulnerable. Princess cuts (four unprotected corners) carry the highest chipping risk and should always be set with V-prongs. Marquise, pear, and heart shapes have pointed tips that also need protective settings. Round brilliants are the most chip-resistant shape due to their continuous curved girdle.
Should I remove my diamond ring during physical activities?
Yes. Gardening, gym work, home repairs, and any activity involving hard surfaces increase the risk of chipping. Even though diamond resists scratching, impact forces along cleavage directions can cause chips, especially at thin girdle edges or near existing inclusions.
Summary
Diamond's supreme hardness — its unmatched resistance to scratching — is real and remarkable. But hardness is not toughness, and "the hardest" does not mean "the strongest." Diamond cleaves along four octahedral planes, chips at thin edges and pointed tips, and varies in hardness depending on crystallographic direction. These are not flaws in the material; they are consequences of the same atomic structure that makes diamond extraordinary. Understanding them means you can choose shapes, settings, and wearing habits that keep a diamond looking as it should — for as long as it should.