The primary reason most 1970 pennies look tarnished while most 1970 quarters do not lies in the difference in their metal composition. Pennies minted in 1970 were made primarily of copper, which is a metal that tends to oxidize and tarnish over time when exposed to air and moisture. This oxidation process results in the formation of a layer of copper oxide, giving the penny a tarnished appearance.
On the other hand, quarters minted in 1970 were composed of a clad metal combination, specifically a layer of copper-nickel alloy over a core of pure copper. The copper-nickel alloy is more resistant to oxidation and tarnishing compared to pure copper. This makes the quarters less likely to develop the same tarnished look as the pennies, even after years of circulation and exposure to the elements.
In summary, the difference in tarnishing between 1970 pennies and quarters is due to the distinct metal compositions used in their production, with copper being more prone to oxidation than the copper-nickel alloy used in quarters.