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Measuring the hardness of a diamond

The hardness of an object determines its resistance to injury in the form of scratches, and diamonds are the hardest of known substances. While we may think of diamonds as fragile, that perceived fragility has more to do with mythologized rarity (despite actually being a common mineral) than susceptibility to surface impact.

The ‘hardness’ of a substance can’t be equated to ‘toughness’, though. An object’s strength involves its resistance to breaking or fracturing. Nothing but another diamond can scratch or cut a diamond, but its moderate toughness means it can still shatter.

Confusion between a diamond’s toughness and hardness has been tested with a hammer. Can a diamond fracture or splinter if hit with a hammer? Yes. Can the surface metal of a hammer scratch a diamond? No.


Measuring hardness

To measure the hardness of a diamond, or any mineral, we use a scale called the Mohs scale, named after its inventor, Friedrich Mohs. This method involves testing one mineral by seeing if another mineral has the capability of producing a scratch on it.

Mohs scale is a 10-point scale, where one is the softest and 10 the hardest. Diamonds are a 10, and 40 times stronger than the mineral in category 9 (Corundum, which can make rubies and sapphires).

Mohs scale reveals some surprises, or unexpected results from how we might normally think of certain substances. For instance, dust, which can corrode surfaces, is ‘hard’ enough to cut glass. Glass, on the scientific scale of things, is actually quite soft.

Mohs Scale operates by comparisons, in four simple possibilities:

  1. If the first substance can scratch the second, the first is harder.
  2. If that first substance cannot scratch the second, the second is harder.
  3.  If the two substances are equal in hardness, neither will be able to produce much of a scratch on the other.
  4. If the first substance can be scratched by the second, but cannot be scratched by a third, the hardness of that first substance is somewhere between the second and third.


Top quality

No other gem or mineral can create an indent on the diamond, thus making it resistant to scratches. A diamond’s structural strength is reflected in its names across various languages. ‘Diamond’ derives from the Greek ’adamas’, meaning ‘unbreakable‘. In an even older language, Sanskrit, the word for diamond is ‘vjra’, which translates as ‘thunderbolt’. The old word carries both the object’s quality and our perception of it.

The hardness of a diamond forms over billions of years inside the earth’s crust. Volcanoes move diamonds to the surface as they sit inside volcanic rock (Kimberlite).

While diamond mines were discovered in Africa in the 1870s, it was in the 1930s that De Beers Company began marketing them as engagement rings. The idea, though, resounds on a metaphorical level: like a genuine relationship, a diamond, something that takes a long time to form someplace deep that then surfaces. A diamond, a hard substance not prone to surface wear, but needing consistent care so as not to break.

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