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How To Perform the Mohs Test

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Calcite crystals have a Mohs hardness of 3.

Calcite crystals have a Mohs hardness of 3. You can't scratch them with a penny, but you can scratch them with glass or a pen knife.

USGS
Identifying rocks and minerals relies heavily on chemistry, but most of us don't carry around a chem lab when we're outside, nor do we have one to take rocks back to when we come home. So, how do you identify rocks? You gather information about your treasure to narrow down the possibilities. It's helpful to know the hardness of your rock. Rock hounds often use the Mohs test to estimate the hardness of a sample. In this test, you scratch an unknown sample with a material of known hardness. Here's how you can perform the test yourself.
Difficulty: Easy
Time Required: mere seconds

Here's How:

  1. Find a clean surface on the specimen to be tested.
  2. Try to scratch this surface with the point of an object of known hardness, by pressing it firmly into and across your test specimen. For example, you could try to scratch the surface with the point on a crystal of quartz (hardness of 9), the tip of a steel file (hardness about 7), the point of a piece of glass (about 6), the edge of a penny (3), or a fingernail (2.5). If your 'point' is harder than the test specimen, you should feel it bite into the sample.
  3. Examine the sample. Is there an etched line? Use your fingernail to feel for a scratch, since sometimes a soft material will leave a mark that looks like a scratch. If the sample is scratched, then it is softer than or equal in hardness to your test material. If the unknown was not scratched, it is harder than your tester.
  4. If you are unsure of the results of the test, repeat it, using a sharp surface of the known material and a fresh surface of the unknown.
  5. Most people don't carry around examples of all ten levels of the Mohs hardness scale, but you probably have a couple of 'points' in your possession. If you can, test your specimen against other points to get a good idea of its hardness. For example, if you scatch your specimen with glass, you know its hardness is less than 6. If you can't scratch it with a penny, you know its hardness is between 3 and 6. The calcite in this photo has a Mohs hardness of 3. Quartz and a penny would scratch it, but a fingernail would not.

Tips:

  1. Try to collect examples of as many hardness levels as you can. You can use a fingernail (2.5), penny (3), piece of glass (5.5-6.5), piece of quartz (7), steel file (6.5-7.5), sapphire file (9).

What You Need

  • unknown specimen
  • objects of known hardness (e.g., coin, fingernail, glass)

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