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The Mohs scale of mineral hardness consists of ten different minerals, but some other common objects can also be used: these include the fingernail (hardness 2.5), a steel knife or window glass (5.5), a steel file (6.5), and a penny.
The penny has always been assigned a hardness of around 3. But we have conducted tests and found this is not true.
The penny has changed in composition over the years since 1909 when the first Lincoln cent was issued. Its composition was specified as 95 percent copper and 5 percent tin plus zinc, an alloy classified as bronze. Except for the wartime year of 1943, pennies were bronze from 1909 until 1962. Pennies for the following 20 years were copper and zinc, technically brass rather than bronze. And in 1982 the proportions were reversed so that pennies today are 97.5 percent zinc surrounded by a thin, thin copper shell.
Our test penny was from 1927, the original bronze formula. When we tested it with a new penny, neither scratched the other, so it's clear that the hardness of pennies has not changed. Our penny would not scratch calcite unless we really bore down on it, but calcite (the standard for hardness 3) scratched the penny.
In the interest of science, we tested a quarter, a dime and a nickel against the penny and against calcite. The quarter and dime were slightly softer than the penny and the nickel was slightly harder, but all were scratched by calcite. We did not experiment with silver coins, however, on a wild hunch, we tested an Indian head penny from 1908 and found that it scratched all the other objects and was not scratched in turn.
So with that exception, all American coins do not scratch clear calcite without a lot of effort, whereas calcite scratches them fairly easily. This gives them a hardness less than 3, that is, 2.5, while an Indian head penny has a hardness greater than 3, that is, 3.5. The Indian head penny had the same nominal composition as the Lincoln penny, with zinc and tin combined making up 5 percent, but we suspect that the older penny had a little more tin. Maybe one penny isn't a fair test.
Is there any reason to carry a penny around when the fingernail also is hardness 2.5? There are two: One, you may have soft nails; and two, you may prefer to scratch a penny rather than your nails. But the practical geologist should carry a nickel instead because in an emergency it can feed a parking meter.