I am not a coin collector. Oh, like most other people, I occasionally save a "wheatie" penny or a new state quarter from my change, and I listen with rapt attention whenever there is a news item about some rare coin fetching thousands or hundreds of thousands of dollars at auction.
So, when I recently spotted a double-headed quarter in my change, my heart skipped a beat and dollar signs danced before my eyes. Alas, my "find" turned out to be what is known as a novelty or "magician's coin" (http://coinworld.com/news/071006/BW_0710.asp), and is worth maybe a dollar or two at a yard sale. (It was a 1998D (Denver Mint) on one side and a 1998P (Philadelphia Mint) on the other, a dead giveaway that it had been manufactured.)
But the whole episode generated a number of questions. For instance, how long had that quarter been in circulation and how many fingers had it passed through, unnoticed, before reaching my pocket? And if such a coin had long escaped detection, how many genuine mint errors might there be in circulation, waiting to be found? Further, just how many mint errors have been produced, and how valuable are they?
Needless to say, I've been assiduously checking all my change since, and a little research has turned up some very interesting answers to my questions.
One very important thing to remember is that there is a difference between mint ERRORS and mint VARIETIES. Mint errors are mistakes in the manufacturing process, such as when a coin gets stuck and is stamped twice ("machine doubling") or is "off-center." Minor mint errors are actually quite common - so common, in fact, that many earn little or no premium. But the more extreme or "spectacular" or rare the occurrence - such as in the case of a true double-headed or double-tail coin or a coin that is way off-center - the more valuable it becomes. For instance, the 2000-P Washington Quarter/Sacagawea Dollar Mule Error, of which there just 10 are known to exist, sold for over $56,000 in 2001. http://www.coinfacts.com/error_coins/mules/washington_sacajawea_mule.htm
Mint varieties, on the other hand, could be considered analogous to editions of books. If the die that stamps a particular coin is improperly made or is altered intentionally or accidentally, it will transfer the same image to any number of coins until the die is again altered or replaced. Thus, a true "double die" coin, such as the famous 1955 Lincoln penny, is a mistake in the die. Some varieties can be extremely rare and valuable since the Mint will fix problem dies as soon as they are discovered. (By the way, the value of the 1955 Lincoln at online auction today? $1200 -1800+!)
And new varieties occur all the time. The 2004-D Wisconsin State Quarter comes in several varieties, with the Extra Leaf Low and Extra Leaf High varieties currently commanding several hundred dollars each.
Of course, as with any collectible, the values of mint errors and varieties depend a great deal on condition, with coins in mint, proof, and/or uncirculated condition fetching the highest prices.
One last thing to remember is that you're much more likely to discover a valuable variety if you go to your bank and ask for rolled or bagged coins that have come straight from one of the mints. (After all, you're not the only one checking circulation coinage!) After checking over all the coins and replacing any potential finds with normal coins of equal value, you can take the coins back to the bank and exchange them for more!
It can take some time and study to be able to distinguish between some machine errors and their similar die varieties, with prices varying widely depending on type, so the following resources are highly recommended should you wish to investigate this area of collecting further.
Good Luck!
Books:
"A Guide Book of United States Coins 2007" (60th Edition), by R. S. Yeoman, Kenneth Bressett (Editor)
http://digbig.com/4nkmq
"The Moneybucks Handbook for Minting Errors & Die Varieties," by Ray Balsbaugh
http://digbig.com/4nkmr
"The Official American Numismatic Association Grading Standards for United States Coins," by Kenneth Bressett, Q. David Bowers
http://digbig.com/4nkms
"The Official Price Guide to Mint Errors," (6th Edition), by Alan Herbert
http://digbig.com/4nkmt
Websites:
CoinFacts.com
http://www.coinfacts.com
Information on different mintages and check out their "Cool Coins" tour.
Coin World
http://www.coinworld.com
Magazine's online site offers news, glossary, events, FAQ's, more.
CONECA
http://www.conecaonline.org/index.html
The Combined Organizations of Numismatic Errors Collectors of America. This excellent site has everything: glossary, abbreviations, articles, events, "Recent Finds", magazine (The Errorscope), links, more!
Coppercoins.com
http://www.coppercoins.com/index.php
Geared to the copper coin/penny collector, this excellent site offers over 8800 photos, a forum, FAQ's, more.
Heritage Auction Galleries - Coins
http://coins.heritageauctions.com/default.php
Check out their Auction Archives with prices and pics from over 946,000 lots.
Mint Error News
http://www.minterrornews.com
Covers recent markets for mint errors/varieties
