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Is That Really a Diamond?

It is amazing how many things people commonly think of as scientific fact which really are false. For example, vultures do not circle over a dying animal. Raindrops are round, not teardrop shaped. One of my favorite misconceptions is the idea that you can identify a diamond because it will cut glass. For this experiment you will need:

  • a glass jar or bottle
  • a variety of rock and mineral specimens

Be sure to use a disposable glass jar, because we will be scratching the glass. Once it is scratched up, you can put it in the recycling bin. Hold the glass firmly in one hand and the rock you want to test in the other. Examine the rock to find an edge with a fairly sharp point. You will use this point to try scratching the glass. Don't press too hard. You don't want to break the glass. Instead, pretend that the rock is a pencil, and press about as hard as you would to write on a piece of paper. If the rock has a hardness of 5.5 or more, it will leave a scratch on the glass. Be sure to rub your finger over the scratch. If it rubs off, it's not a scratch. Sometimes a soft mineral will rub off on the glass and the streak will look like a scratch.

Once you find a rock which will scratch the glass, you have a diamond, right? No, unfortunately not. The hardness of minerals is measured using the Moh's hardness scale. This scale ranges from 1 to 10, from the softest mineral to the hardest.

Moh's Hardness Scale
1 - Talc
2 - Gypsum
3 - Calcite
4 - Fluorite
5 - Apatite
6 - Orthoclase
7 - Quartz
8 - Topaz
9 - Corundum
10 - Diamond

On this scale, glass is 5.5. That means that any mineral that is harder than 5 1/2 will scratch glass. If you believed that only a diamond would scratch glass, then you might be convinced that a common quartz crystal was a fabulous diamond. Try a variety of rock and mineral specimens to see which ones will scratch the glass.

There are other items around your house which you can use to test the hardness of the mineral. A piece of copper has a hardness of three. Your fingernail has a hardness of about 2.5. If you have specimens of some of the minerals from the Moh's scale, you could use them too. A mineral which will scratch a piece of calcite has a hardness of 3 or more. Keep in mind that two things of the same hardness will scratch each other. For example, copper will scratch calcite and calcite will scratch copper.

Testing hardness is just one of the tests used to identify unknown minerals. If you are interested, go to the library and get a good book on minerals. It will have information on identifying minerals by how the break, by the color streak they leave on a tile, by the shape of their crystals and other tests.

You can buy mineral specimens in the gift shop at most museums, but you can also find your own. You can probably find a piece of minerals such as quartz or calcite in the gravel of your driveway. Once you start to look for them, you will notice rocks and minerals all around you. Just be careful. It might lead you into a wonderful hobby.