I found a 2009 penny, and I am confused, cause it's really brown. I thought that a coin that's less than two years old would still be shiny.
One of my sons was able to turn an un-circulated president dollar coin brown within a few days. lol For whatever reason, he likes to hold his coins all the time and I guess the moisture from his hands (he's a young hyper kid) turned it colors. Quite amazing to see a coin change color like that in a short amount of time.
Time plays a role, but as was alluded to above, environment plays a more fundamental role into why your 2009 coin is toned. Do this experiment. Take three new pennies and a fresh box/pack of matches. Place one coin inside the matchboxes. Place the other two of them on a shelf in your bedroom -- a bookshelf is fine. Every day for a two weeks, simply handle ONE of the coins. Handle the same one every day. Every time when you put the coin back look at the color difference between them, and at the end of two weeks also compare the coin in the matchbox. You will have the answer you seek. As an interesting sidebar, take the three pennies and put them in a pocket of a piece of clothing (preferably one you can close completely). Now, wash the clothes and pennies - a full wash cycle. Look carefully at what results. You might notice a blue coloration that appears only in certain areas of the three coins that were "browned" before, but do the experiment yourself and observe carefully what you find. Now place the same coins in the dryer with the wet clothes (you might even try no, low, and high heat) and compare again.
Most people seem to think that natural toning requires long periods of time. It doesn't. Under the right conditions 100% natural toning can occur in a very short period of time. Under other conditions, it can take a 100 years. No, it's not extremes. It just various conditions. Different storage methods and different environmental conditions, even very tiny ones, can have a dramatic effect.
I would bet on that or MS-70. Some type of detergent is the only thing I'm aware of that causes that type of toning.
Interesting thoughts on color... I have a penny barrel that was begun as a pile of change about 30+ years ago, and notice that the cents that still retain some luster range in color from strawberry red tint to green or blue. I guess the color of tone on these cents depended more on what they came into contact with during their short stint in circulation vs just being in containers for 30 years? Otherwise, one would think all of the coins would tone exactly the same, if at all. Correct??
What you have to realize is that coins, all coins, come into contact with various things whether they are stored Uncs or in circulation. You cannot keep coins from what's in the air. And since the air is different in every single town, and yet different again in every single home in that town - you have an endless list of possible things that the coin has come into contact with. And that doesn't even take into account the various storage methods, different levels of humidity, and different temperatures. All of these things play a part in how a coin tones. Then you have the coin itslef. And since no two coins are identical, no two coins will tone exactly the same even under the same conditions. They may be similar, but not the same.
