Most people are familiar with the colorful little "stickers" that appear on the top right corner of envelopes mailed to us: Stamps. Some of us may even have had stamp collections when we were kids, since forgotten in a box in a closet or attic. A few, of course, continue collecting throughout their lives an sometimes end up owning some very valuable items.
Just how valuable can a stamp be? There are a few "great rarities" in the stamp collecting world- these are items that have sold for more than one million US dollars, generally at public auction. In general, such "gems" are either one-of-kind, or one of just a few few known examples. Generally, they tend to date back to the early days of postage stamps, in the mid- to late 1800's.
Many of the world's most valuable stamps are well known to collectors, as they tend to have famous histories. There is Sweden's 3 Skilling Bco yellow, believe to be a unique "mistake" in which a single clich of the three Skilling value was mistakenly put into a sheet of Eight skilling stamps and printed in the yellow color used for those. Then there is the British Guiana (now Guyana) 1 cent magenta from 1856, also believed to be a unique stamp, and also an "error" in which one stamp was mistakenly printed as 1 cent instead of 4 cents. The most valuable US stamp is the 1c Benjamin Franklin stamp issued in 1868, known to collectors simply as "The Z-grill" for a particular embossing in the stamp paper. Whereas it is not unique, a copy was most recently traded for about three million US dollars.
Perhaps the world's most famous valuable stamp is the US "Inverted Jenny" from 1918. The stamp was printed in two colors, which required two passes through the printing press, one to print carmine, one to print blue. A single sheet of 100 stamps was accidentally turned upside down when the second color was printed, giving the impression that the airplane is upside down. Although 100 examples are believed to exist, the stamp's well known history means that copies typically sell for $250,000-$300,000.
Stamps don't HAVE to be old to be valuable. As recently as 1980, a "modern error" caught the attention of stamp collectors when some CIA employees bought a sheet of $1.00 stamps featuring a colonial rush lamp candle holder, in which the candle holder and text was printed inverted from the candle and flame. Currently, the whereabouts of 95 of the 100 stamps is known, and these stamps typically sell for about $15,000.
The world's first postage stamp was issued by Great Britain in 1840. Known as the "Penny Black," it marked the beginning of "uniform rate" postage in a country- previously, the cost to send a letter depended on the distance it was carried. Although millions of "Penny Blacks" were printed and sold for one English penny (about two cents) an unused copy in nice condition today sells for close to $3000.00. Not bad, for a two cent investment!
The term "valuable" of course has different meanings to different people. Whereas most collectors are unlikely to find a great rarity in an attic, it's not uncommon for old collections from estate sales or grandma's attic to turn up individual stamps worth $100's, or even $1000's. I have personally found a rare stamp in what collectors call a "box lot" that I subsequently sold on eBay for over $2000. The entire box of albums, envelopes and loose stamps cost me $50.
So, just what makes a stamp valuable? It's usually a combination of scarcity, condition and collecting popularity of the country it came from. Although not always true, unused (or "mint") stamps are generally more valuable than used stamps. Similarly, the age of the stamp also affects the price in most cases- simply because the older a stamp issue gets, the more examples of that stamp gradually disappear (in attic clean-ups and the like) or are damaged by people handling them. That said, there are certainly modern stamps- usually with some kind of printing error- that turn out to be very valuable.
In any case, part of the fun of the stamp collecting hobby is definitely its "treasure hunt" aspects. You just never know what you might find in the next box of old stamps you run across in your great uncle's attic!