For collectors, a general rule of thumb is as follows: the rarer a collectible, the greater its potential value could be.
This is largely due to supply and demand driving the collectibles markets, combined with the indescribable feeling of holding a one-of-a-kind piece of history in your hands.
At the same time, there is a worldwide desire among collectors to own these singular pieces - which drives buyers to purchase some truly bizarre collectibles.
Here is Paul Fraser Collectibles' guide to the top 10 most unique valuable collectibles. And each of them is 'unique' in more than one sense of the word...
#10 X-rays of Einstein's skull - $38,000
Two X-rays from 1945, each revealing the genius behind the theory of relativity's skull, sold for $38,000 at a Julian's auction in 2010.
Showing both a frontal and profile view of Einstein's skull, the X-rays held additional interest for collectors having been originally taken by Dr Gustav Bucky. Bucky was a forerunner in radiograph technology who collaborated with Einstein to invent the Automatic Electric Eye camera in 1937.
Perhaps not surprisingly, the lot far exceeded its $1,000-2,000 pre-sale estimate. X-rays of Marilyn Monroe and John F Kennedy have also appeared on the markets.
#9 Neil Armstrong's hairdresser's scissors and comb - £35,000
In May 2005, the first man to step on the moon's surface became involved in a legal battle with his barber, Marx Sizemore, after finding that the barber was selling off cuts of his hair to memorabilia collectors.
Armstrong threatened legal action unless the hair was returned, or the sale proceeds donated to charity.
The scissors & comb used by the barber, along with 25 strands of the astronaut's hair, are currently for sale at Paul Fraser Collectibles priced at £35,000.
Armstrong stopped signing autographs for the public in 1994 after he found out that many forgeries were selling for large amounts of money. His autograph is now considered to be the most valuable autograph of any living person.
#8 The 'earliest pair' of Levi's jeans - $60,000A pair of Levi Strauss & Co (USA) 501 jeans were sold to a Japanese collector for $60,000 on eBay in 2005.
The garment, made in 1890, is the earliest pair of Levis to have ever appeared at auction. Levi Strauss & Co first began making denim overalls only two decades earlier, in the 1870s.
Today, the brand has more than 470 stories around the world and remains one of the worlds' most iconic jean manufacturers - iconic enough for one collector to pay a five-figure sum to own this unique part of the firm's legacy.
#7 Lee Harvey Oswald's coffin - $87,000How's this for a 'niche market'? The coffin in which Lee Harvey Oswald was originally buried was auctioned for $87,000 in November 2010. Bids began at a minimum $1,000, before rocketing upwards.
While macabre, the coffin is undoubtedly a key part of the United States' history. The body of President Kennedy's assassin was exhumed in 1981 to disprove the suggestion that his body was in fact that of a Russian spy.
When returned to the ground, Oswald's body was placed in a new coffin - hence this opportunity for a lucky buyer to acquire his old casket.
#6 Stairs from the Eiffel Tower - €105,400Perfect for that loft conversion... An original spiral staircase from the Eiffel Tower sold for €105,400 at Paris's Drouot auctioneers in December 2009.
The iconic Eiffel Tower was considered an eyesore when it was first erected as the entrance to the World's Fair in Paris in the late-19th century. The 40 steps, dating from 1889 when the tower was first opened, are 7.8 metres high.
They were removed from the tower during renovation work in 1983, and later appeared at Drouot's 2009 auction alongside various other Parisian architectural treasures.
#5 Napoleon's penis - $100,000Its whereabouts were unknown for decades, but Napoleon's penis finally appeared in 1916.
Descendants of the priest who administered the last rites to Napoleon in 1821 sold an item claimed to be the emperor's penis to a rare book company.
The appendage, believed to be between an inch to an inch-and-a-half in length, was last seen at auction in 1977. A professor at Columbia University got his hands on it for $3,000.
Napoleon's penis has since been passed down through the family. Believe it or not, the professor's daughter recently turned down an offer of $100,000 to part with it.
#4 Apollo 11 navigational chart - $218,000This piece is a historically important as it is unique, having been used by Apollo 11 astronauts Neil Armstrong and Buzz Aldrin to determine their exact position on the lunar surface, just after their historic 1969 moonlanding.
It appeared at Bonhams' Space Sale in New York in 2009 and, as one of the few flight devices returned from the lunar surface to be available on the market, was a major highlight in the sale. In the end, it sold for $218,000.
"This star chart was the single most critical navigational device we used while on the Moon," wrote Aldrin in an accompanying letter. You can't beat provenance like that.
#3 Unique jewelled caterpillar - £300,000Jewels made to resemble butterflies are not uncommon. Yet the butterfly's less-celebrated precursor, the caterpillar, is often overlooked by jewellers.
Although this wasn't the case at a Sotheby's rare jewellery auction in 2010. Among the highly-valued lots for sale was a rare gold, enamel, diamond, pearl, ruby, emerald and turquoise "Ethiopian Caterpillar".
Thought to have been made by Henri Maillardet around 1820, the caterpillar is even more remarkable for featuring a special knurled wheel which gives it 'a life-like motion'. It sold for £300,000.
#2 The only Billy the Kid photo in existence - $2.3mThe legend of notorious outlaw Billy the Kid still captivates people 130 years after his death. So it's no surprise that a one-of-kind memorabilia piece directly linked to the gunslinger drew bidders from around the world when it auctioned in June 2011.
Brian Lebel's 22nd Annual Old West Show & Auction in the US boasted the only extant photo of outlaw Billy the Kid among its collectibles for sale.
The Kid posed for the picture at a saloon in Fort Sumner, New Mexico. Unbeknownst to him, the photograph would be pored over for more than 100 years. After furious bidding, the photograph sold for a remarkable $2.3m.
The Kid was later shot and killed by Sheriff Pat Garrett in 1881 at the age of just 21.
#1 Marilyn Monroe's Seven Year Itch Dress - $5.6mAsk most people to name Marilyn Monroe's most iconic film moment, and there would be one obvious contender: the scene in the 1955's The Seven Year Itch when Monroe dances above a subway grate.
The dress emerged as the highlight in the auction of Debbie Reynolds' collection at a Los Angeles auction held by Profiles in History in June 2011. Reynolds, herself a star of the silver screen, acquired much of her collection in a 1970s auction held by MGM studios.
Profiles in History's sale featured costumes worn by Laurel and Hardy and Audrey Hepburn, among others, Marilyn's Seven Year Itch was the real heard-turned. In the end it sold for an incredible $5.6m World Record price.