Diamonds: Hard Yet Brittle—Why They’re Not Unbreakable

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Diamonds are the hardest naturally occurring material on Earth. The name derives from the Ancient Greek word adámas, meaning “invincible” or “unbreakable.” Yet a strike from a steel hammer can shatter a diamond into tiny shards.

How can something be so hard and yet so brittle? The confusion stems from how “hardness” is defined. In everyday language, hard means resistant to bending, cutting, or breaking. In materials science, hardness quantifies a material’s resistance to permanent, localized deformation. Scientists classify hardness into three types:

• Scratch hardness: measured by dragging a harder material across the surface and noting the groove depth.
• Rebound hardness: determined by dropping a hammer‑like tool tipped with a harder material and measuring the bounce height.
• Indentation hardness: obtained by pressing a harder material into the surface under a fixed force and measuring the indentation depth.

Because no material is harder than diamond, it always deforms the tool rather than being deformed itself. In fact, diamonds set the upper limit on most hardness scales. On the Mohs scale, diamond scores the maximum 10.

Low Toughness Explained

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While hardness and toughness are often used interchangeably in everyday speech, they are distinct in materials science. Hardness measures resistance to shape change; toughness measures a material’s ability to absorb energy before fracturing. By this definition, rubber is far tougher than diamond. A rubber ball can absorb a sledgehammer blow without breaking, whereas a diamond will shatter into countless fragments upon the first impact.

Diamond’s low toughness is rooted in its crystal structure. Each carbon atom is covalently bonded to four neighbors, forming a rigid lattice that resists surface deformation. However, this same rigidity makes the lattice vulnerable to shear stresses. When a shock propagates through the crystal, it encounters cleavage planes—preferred fracture directions—that allow bonds to break rapidly, causing the diamond to crack like a house of cards.

Cutting Diamonds Without Breaking Them

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The delicate art of diamond cutting leverages the material’s hardness while respecting its brittleness. Since no tool is harder than diamond, cutters use diamond‑tipped saws or lasers to score the stone at the desired angles. A steel cleaver then forces the score line, splitting the diamond along its cleavage plane. Steel’s higher toughness allows it to withstand the cutting force without fracturing.

After the initial split, cutters polish the facets using successive diamond abrasives. The diamond is held in a cemented holder, with the unwanted edge exposed. That edge is rubbed against a second diamond to remove material, creating a flat face. The final polish employs diamond powder sandpaper, ensuring a flawless finish.