The Mohs Hardness of Coins

Is a Penny Really Hardness 3?

lincoln and indian-head pennies
Lincoln cent: hardness 2.5; Indian head cent: hardness 3.5. Photo (c) Andrew Alden, licensed to (fair use policy)


The penny has always been assigned a hardness of around 3. But I have conducted tests and found this not to be true.

The penny has changed in composition over the years since 1909, when the first Lincoln cent was issued.


My test penny was from 1927the original bronze formula. When I tested it with a new penny, neither scratched the other, so it's clear that the hardness of pennies has not changed. My penny would not scratch calcite unless I really bore down on it, but calcite (the standard for hardness 3) scratched the penny.

In the interest of science, I 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.

I did not experiment with silver coinshowever, on a wild hunch, I tested a Indian head penny from 1908 and found that it scratched all the other objects and was not scratched in turn.


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 I suspect that the older penny had a little more tin. But 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? I think 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.

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Your Citation
Alden, Andrew. "The Mohs Hardness of Coins." ThoughtCo, Feb. 28, 2017, Alden, Andrew. (2017, February 28). The Mohs Hardness of Coins. Retrieved from Alden, Andrew. "The Mohs Hardness of Coins." ThoughtCo. (accessed November 20, 2017).