Priceless Paperweight

So there's this MacGuffin right—perhaps one that The Hero and company must find in order to stop the The End of the World as We Know It, or maybe something that explains all they need to know to solve some problem at hand. Somebody goes searching for it, looking under every rock, solving every puzzle, and maybe even going all apeshit on someone who might know where the MacGuffin is. But eventually they find the MacGuffin, and discover that this priceless, important, amazing artifact/object/spear/war-souvenir/whatever is being used...as a paperweight.

Priceless Paperweight, therefore, is when said important and/or priceless artifacts or other objects are being used for ridiculously mundane purposes, like paperweights, footstools, or a pretty decor for their bookshelf. Oftentimes this is Played for Laughs, usually by showing The Hero's or some other person seeing the usage's shocked reaction to such a stupid way of using something so valuable.

Oddly enough, this is sometimes Truth in Television, as noted by some examples below.

Compare with Grail in the Garbage, Hidden in Plain Sight, Mundane Utility, Useful Book, and Worthless Yellow Rocks.

Examples of Priceless Paperweight include:

Anime and Manga

In Yu-Gi-Oh! Pegasus is shown via Flash Back discovering the Pharaoh's tomb during a journey to Egypt. His next move? Using it to make Egyptian God Cards to be sold to the masses.
- Lampshaded in Yu-Gi-Oh the Abridged Series.
- Then again, it's justified in this case; he was manipulated into making the cards by the tomb's protector spirit as a conduit to bring the tomb's Sealed Evil in a Can to the modern world. And, as we see in the Battle City arc, he at least donated some of his findings to museums.
The Book of Darkness in Magical Girl Lyrical Nanoha is a powerful Artifact of Doom capable of granting world-destroying powers to any mage. When it landed in Hayate's possession, she took good care of it not because she knew what it was, but because it looked really pretty in her bookshelf.
The Pursuit of Harimao's Treasure: When Zenigata finds the bronze bear statue lying around Lupin's apartment, he isn't sure what to make of it. So he uses it to weigh down the lid on his ramen noodles, while they steam! Not realzing that statue is actually part of a set needed to unlock the fabled Harimao treasure.

Comic Books

One of the early Power Rangers comics had Kimberly use her power coin as a coaster for her soda. She then forgot where she put it and nearly got the team killed for it when she couldn't show up for the Megazord activation.
In a Don Rosa story, Scrooge McDuck seeks to restore one of the lost Wonders of the Ancient World, the Temple of Artemis at Ephesus, in order to decipher the location of the treasure-trove of King Croesus (who bankrolled the construction of the temple) from writing scattered across the columns. Most of the pieces of the columns were scattered across the world when invading Goths tore down the temple and carted off the pieces as trophies. So, Scrooge travels all across the world, seeking out the lost pieces in a Montage - finding several of them being used in mundane ways.
- Another example from Rosa's stories is one where Scrooge finds a plaque and a document that proves the hill his money bin stands on was never a part of the United States, having been given directly to the founder of Duckburg by the Spanish. He declares it a sovereign country but this ends up causing him a lot of trouble (the city cuts off his electricity and the Beagle Boys invade and take over since police jurisdiction does not extend to foreign countries). In the end Scrooge joins his country back to US and uses the plaque as a tray for birdseeds.
In a relative example, one Far Side comic featured an elephant who had recently lost his foot to poachers. The elephant was understandably upset to learn that his foot had been turned into a novelty wastebasket.

Film

Iron Man 2: Tony Stark uses Captain America's prototype/unfinished shield to prop up what he is currently working on.
Discussed in Raiders of the Lost Ark where Indy's rival Rene Belloq seems to think a lot of what archaeologists like himself view as valuable are Inversions of this Trope:

Belloq: Look at this. (Shows Indy his watch.) It's worthless. Ten dollars from a vendor in the street. But I take it, I bury it in the sand for a thousand years, it becomes priceless... like the Ark. Men will kill for it. Men like you and me.

Literature

One of Neil Gaiman's short stories, "Chivalry", involves the holy grail being found by an old lady in a thrift store. The ancient knight who comes in search of the grail discovers that she knows exactly what it is, and keeps it for no greater reason than because it looks nice on her mantle.
The Prince and the Pauper, the court officials keep asking the false Prince, Tom Canty, about the whereabouts of the Great Seal of England, hidden by the real Prince Edward Tudor just before their escapades began. Despite repeating question the disguised Tom has no idea what they're talking about. In the end, Edward returns and proves his true identity by at once pointing out what it was. Tom admits he'd been using every day as a nutcracker.
In Life The Universe And Everything, the pieces of The Wikkit Gate have all been scattered throughout the galaxy and are mostly being used for mundane things.
Inverted in The Ingenious Hidalgo Don Quixote of La Mancha, where the main character takes genuinely mundane things for this.
In Nomads of Gor, the (last) egg of a male Priest-King is hidden amongst the Wagon Peoples. Kamchak of the Tuchuks uses it as a footstool.
In the Safehold series, the Wylsyn family has an artifact known as the Key, which was handed down to them by the Archangel Scheuler. It looks like a well-polished stone. Due to its unremarkable appearance, generations of Wylsyns have hidden the key by displaying it on their desk and using it as a paperweight.

Live-Action TV

One guy in The Sarah Connor Chronicles finds an abandoned Terminator head and decides to hold onto it, apparently just because it looks cool. But then its body comes back looking for it and, yeah.
This trope happens all the time with Kryptonite on Smallville, whether it's necklaces or class rings.
Buffy the Vampire Slayer:
- Jenny Calendar needed an Orb of Thesulah for a ritual to give Angel his soul back. A guy who sells magic supplies mentions to her that there isn't much call for them, since the rituals of the undead were lost, but he did sell a couple as new age paperweights last year. A few episodes later, Giles reveals that he has one in his office, where he's been using it as a paperweight.
- At the beginning of season seven, Andrew kills Jonathan using a cursed ceremonial dagger, using his blood to open the Seal of Danzalthar. Later, after spending several episodes as a guest/hostage of the Scoobies, he relates this to Buffy and she demands he give her the dagger, as it might be useful. Where did he put it? The cutlery drawer in the kitchen! “I washed it first!” he insists.
The Harlem Globetrotters on Gilligan's Island has Gilligan using an energy-producing MacGuffin as a lamp.
The BBC show Cash in the Attic is basically this trope: people need money for some expense, and get the show's antiquarian to inspect their valuables for the occasional diamond in the rough. They tried the same thing in the States, but the show wasn't nearly as possible, most likely because America as a country isn't as old as the United Kingdom.

Tabletop Games

Basic Dungeons and Dragons adventure IM3 The Best of Intentions. One of the magical artifacts the PCs need to acquire is being used by a giant bacteria to shield his underside from unpleasant drafts.
- In the Dungeons and Dragons setting Scarred Lands, The pantheon of Gods managed to slay the titan Kadum (aka The Mountainshaker). the mighty titan's source of power was his heart, so the goddess Belsameth (aka The Slayer) ripped it out... and has since used it as a footstool.
In the Old World of Darkness, there was a Villain Sue who just got mightier and mightier, until he at last died in a Mage adventure. When people started speculating that he would come back as a wraith, the publishers declared that he DID become a wraith, but was captured by the evil souleating empire. They recycled his soul for raw material, and used that particular unit of soulsteel to make an ashtray for one of their bureaucrats.
Magic: The Gathering has the Salvation Sphere. It is one of the pieces of the Legacy, a group of ancient artifacts that that the Weatherlight crew must collect so they could save Dominaria from the Big Bad, Yawgmoth. Most players probably won't recognize it by its original name though, since it's more popularly known as, uhh... Squee's Toy.

Video Games

In Superhero League Of Hoboken, a few missions have you tracking down priceless artifacts from the "Before-Time". These include a diet book (priceless pre-end of the world knowledge!) and a Frank Sinatra tape. Two of the artifacts, however, are being rather mundanely used: George Washington's Museum's Souvenir Rack, which is made of gold and being used as an ostentatious coat rack, and a VHS video tape, which is being used to stabilize a wobbly table.
At the conclusion of the Firewalker mission in Mass Effect 2, Shepard and his/her crew come across an artifact: a metallic sphere implied to be a data storage device of some sort. After confirming the artifact is inert and harmless, Shepard uses it as an ornament for his/her coffee table on the Normandy.
The NES port of Ikari Warriors hangs an unintentional lampshade on this. A silver-colored Palette Swap of the hidden Gold item is named the "Paperweight". It's worth 5000 points just like the Gold.

Web Comics

The trope is discussed in this strip of Wapsi Square when Brandi mentions that the portal cloth they just acquired wouldn't make a very good table cloth.

Western Animation

In an episode of Kim Possible ( which ends up just being a dream), Ron finds an old book written during the founding days of Middleton being used to support a wobbly chair in his kitchen. The book ends up having (most of) the answers to the mystery of Kim's disgraced ancestor.
My Life as a Teenage Robot: One episode has Jenny, Brad, and Tuck looking for Jenny's robotic body parts being scattered around the world, most of which are being used as this trope.
In the South Park episode "Mecha-Streisand," Cartman and Kyle discover an ancient artifact capable of giving its user ultimate power. To them, however, it's a cool glowing triangle and they fight over the right to either let it sit on Kyle's dresser or in Cartman's shoe.
An episode of Beetlejuice has the main character subjected to something similar to what happened to Jenny. In this case, Lydia finds that one of the bad guys is using his legs as TV antennae.
DC Super Hero Girls,
- In "Enter Night Sting", Kara mistakes Karen's nano-oscillation scope (a tool used for building Applied Phlebotinum devices) for a toothpick. (Gross...) It is then shown that she has "borrowed" many other things from Kara and uses them like this, which becomes an issue when Karen's future self appears, stating that an incident like this has caused an apocalyptic disaster which Karen has only an hour to prevent...
- In another episode, Diana is so annoyed by Harley's shenanigans that she goes to blow off steam by sharpening her sword, inadvertently using the ear from Batman's cowl (which Harley had given Babs, and is now Babs' cherished keepsake) as a whetstone. Naturally, she breaks it as a result, and must cooperate with Harley to replace it before Babs finds out.

Real Life

There's an old story about two boys in Africa who were found by explorers playing catch with a stone. Upon closer inspection, the stone was actually an enormous uncut diamond.
Two Egyptologists found a 7th century Egyptian statue being used as a bike rack in a museum in Southampton.
Mummies were once a lot more common in Egypt, since mummification had previously been a very popular form of burial. Before the world fully appreciated the scientific and historical significance of mummies, many of them were used to make medicine (not something they're good for). There are also stories of them being used as fuel for locomotives, but the jury's still out on the truth of that.
The first gold rush in the United States was sparked when it was discovered that the Reed family was using a 17 pound gold nugget found on their property as a doorstop.
Recently^[when?] a pair of British siblings^[who?] discovered that their late father's property included a small Chinese porcelain statue that had been just sitting around without any significance, that had the value of 50 million pounds!