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How Do Scientists Rank Minerals from Soft to Hard, 1 to 10?

Category: Nature

You can scratch some minerals with a fingernail. Others, like diamond, can scratch almost anything. The short answer to how scientists rank them: they let the minerals scratch one another, and the winner counts as harder.

That simple test became a ruler from 1 to 10, called the Mohs scale. Let us see how it works, why a gap of one step can mean very different things, and what "hard" does not tell you.

The Rule: Whatever Scratches the Other Is Harder

Take two minerals and drag a corner of one across the other. If one leaves a scratch, it is the harder one. The one that gets scratched is the softer one. The U.S. National Park Service describes hardness in just these terms: a mineral's relative resistance to scratching [1].

The great thing about this test is that it needs very little equipment. Comparing minerals by scratching is much older than the scale itself. The National Park Service says the method has been used as far back as 300 BC and was mentioned by the Greek writer Theophrastus in On Stones [1]. That is an early example of comparing hardness, not the Mohs scale, which came much later. The scale is named after the German mineralogist Friedrich Mohs, who put the idea into numbers [2].

Sources give two dates for this. Wikipedia and geology.com say Mohs introduced the scale in 1812 [2] [3]. A Scientific American history blog says he published it in his textbook Grund-Riß der Mineralogie in 1822–1824 [4]. They may be describing different books, so this article gives both rather than choosing one.

The Ten Reference Minerals, from Talc to Diamond

Mohs chose ten minerals as reference points for the scale. The softest is number 1 and the hardest is number 10 [1]:

  1. Talc
  2. Gypsum
  3. Calcite
  4. Fluorite
  5. Apatite
  6. Orthoclase (a kind of feldspar)
  7. Quartz
  8. Topaz
  9. Corundum
  10. Diamond

To rank a mystery mineral, you see which of these it can scratch and which can scratch it. If it scratches fluorite (4) but is scratched by quartz (7), its hardness is somewhere in between.

Where Everyday Objects Fall on the Scale

You do not need a mineral kit to get a feel for the numbers. The National Park Service lists a fingernail at about 2.5, a copper penny at 3.5, a knife blade or a piece of glass at about 5.5, and a steel nail at about 6.5 [1].

That tells you something useful. A fingernail (2.5) is harder than gypsum (2) but softer than calcite (3), so a fingernail should be able to scratch gypsum but usually not calcite.

Treat these as rough guides. Geology.com gives somewhat different ranges for the same objects, such as 2 to 2.5 for a fingernail and 5 to 6.5 for a knife blade, and warns that values for many common items vary a lot [2].

Why the Steps from 1 to 10 Are Not Equal

In this article, "hardness" means resistance to scratches, not how hard something is to break. It is natural to picture the scale as a staircase with ten identical steps. It is not. The numbers only give the order, from softest to hardest. They say nothing about how big each difference is.

Here is the evidence. Wikipedia reports a different kind of measurement, called absolute hardness, in which corundum (9) comes out about twice as hard as topaz (8), and diamond (10) about four times as hard as corundum [3]. A simple way to picture it is a staircase where the last step is far taller than the rest.

Other measurements give other ratios. Geology.com's table, which uses a method called Vickers hardness, puts diamond at roughly five times corundum [2]. The exact figure depends on the method, but both show a big difference between corundum and diamond, even though they are only one number apart. So you cannot say that a mineral at 6 is "twice as hard" as one at 3. The numbers are ranks, not amounts.

Hard Is Not the Same as Hard to Break

The Mohs scale measures only one thing: how well a mineral resists being scratched. Geology.com reminds readers that the test shows resistance to scratching and nothing more, and notes that hard materials often break when stressed [2]. In everyday words, a very hard mineral can still shatter if you hit it. Hardness and toughness are different properties.

The scale also has limits at the top. Geology.com reports that researchers have found that wurtzite boron nitride and lonsdaleite may be harder than diamond, but these are extremely rare [2]. Because most modern abrasives fall between 9 and 10, researchers later extended the scale to make more room between very hard materials, giving diamond a score of 15 [5].

Try It Yourself: A Fingernail Scratch Test at Home

Use your fingernail as a tester. With an adult's approval, gently rub it on a pencil eraser or a candle. Compare the rubbed spot with an untouched spot: did you scratch the surface, or just leave a smudge? Sort the items into "scratched" and "not scratched."

Remember what the result means. If your nail scratched it, the object is softer than your nail. If it did not, the object is as hard as your nail or harder, and the test cannot say which. Use only your nail, not a knife or anything sharp, and leave valuable or breakable things alone.

To keep exploring, the National Park Service page on the Mohs scale [1] and the geology.com page [2] are good places to start. Next time you see a gem, you might wonder where it sits on the ladder.

Sources

  1. U.S. National Park Service, "Mohs Hardness Scale" https://www.nps.gov/articles/mohs-hardness-scale.htm (Read in full. Source for the ten minerals, the everyday object values, the definition of hardness, and the 300 BC comparison. It gives no year for the scale and does not discuss unequal steps.)
  2. Geology.com, "Mohs Hardness Scale" https://geology.com/minerals/mohs-hardness-scale.shtml (Read in full. Source for the 1812 date, the object ranges, the note that steps are not uniform, scratch resistance versus breaking, and the materials possibly harder than diamond. The roughly fivefold Vickers ratio was worked out from its table, which does not state a ratio. Geology.com is a commercial education site, not a government agency.)
  3. Wikipedia, "Mohs scale of mineral hardness" https://en.wikipedia.org/wiki/Mohs_scale_of_mineral_hardness (Read in full. Used only for the 1812 date and the absolute-hardness ratios of about 2 and about 4, which are not given by the park service source. Wikipedia can be edited by anyone.)
  4. Scientific American, "When Rock Classification is not hard anymore, thank Mohs Scale of Hardness" (History of Geology blog) https://www.scientificamerican.com/blog/history-of-geology/when-rock-classification-is-not-hard-anymore-thank-mohs-scale-of-hardness (Read in full. Source for the 1822–1824 publication in Grund-Riß der Mineralogie. It does not discuss how the ten minerals were chosen or equal spacing.)
  5. Wikipedia, "Scratch hardness" https://en.wikipedia.org/wiki/Scratch_hardness (Read in full. Source for the later scale that gives diamond 15 and the note that most modern abrasives fall between 9 and 10. Wikipedia can be edited by anyone.)