As Doug Winter illustrates in Coin Week, rare gold coins.com, there are all kinds of reasons that coins are especially valuable or rare. Sometimes coins are rare because their condition is so extraordinary. Other times, coins are rare just because there's not that many of them around. The first are called condition-oriented rarities and the second are called absolute rarities. There is even a third category which features coins that have both attributes. These are coins that have both things going on - they are both in extremely good condition and or the date is extremely rare as well. There are very few of these and there are even fewer of these that have managed to survive.
Other things that go into making a coin particularly valuable are very rare date. For whatever reason there were few of them in the first place and consequently very few of them are still left. And lastly, there are certain kinds of strikes that are quite rare - some are limited editions and others might be mistakes. http://www.coinweek.com/
Collecting coins as every coin collector knows is incredibly fun as well as possibly profitable. Many investors enjoy combining their collecting bug but with their investment strategy and having the best of both worlds. Many times collecting coins becomes a hobby that starts in childhood and extends into the retirement years.
Wondering what a rare coin is worth? Well, they're worth a lot. For example, the 1861 Paquet Reverse sold for $1,610,000 in 2006. There are only two such coins known to exist. Now, that my friends is rare indeed. For the rest of us mere mortals, there are the standard issue gold coins… that are collectible, and fairly valuable, but not rare. Not yet, anyway.