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Norcross, GA 30093-3059

"Hardness made harder"
and you thought I was a nerd?
Julian C. Gray, P.G.


The Moh's scale of hardness is familiar to all. An interesting item by J. Alex Speer of the Mineralogical Society of America appeared on page 273 of the July/August 2003 issue of Rocks and Minerals magazine. This technical note refines one of the standard points on the Moh's hardness scale.

Most mineral identification books list pennies as having a hardness of 3. Alex reports that in 1983 the U.S. Mint switched to a softer copper-alloy. After 1983, pennies were minted from copper-plated zinc. During 1983 both copper and copper-alloy pennies were minted, so you cannot be certain of their composition. Therefore, 1983 pennies may have a hardness of either 2 or 3. Pennies with a 1984 or younger date have a hardness of 2! In order to use pennies as a hardness 3 standard, they must bear a mint date of 1982 or older. I guess you could have two pennies in your hardness kit to test, one for H = 2 and another for H = 3.

Thanks Alex. This is why I invested in a set of hardness points.



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