You won't believe some of the fascinating objects that have been washed up on beaches, stumbled upon in backyards or discovered in dusty attics. Here are 20 of the most intriguing.

In 2011, San Franciscan Steven Hoffer re-discovered a heap of dust-covered boxes in his parents' attic that for some unknown reason he had completely forgotten about. On closer inspection, he realised that he was the proud owner of 50 pristine first-edition Star Wars toys worth a very respectable $10,000 (£7,650).

In 2013, a New York family made the sort of discovery most of us can only dream about. Rummaging around a neighborhood garage sale, they chanced upon a fairly ordinary-looking white bowl but decided nonetheless to buy it for the princely sum of $3 (£2.30). It later turned out to be a 1,000-year-old Chinese treasure and was sold for $2.2 million (£1.7m) at auction.

Hunting for cheap canvases one day in 2012, Beth Feeback, a hard-up artist from North Carolina, made a beeline for her local charity shop and snagged a couple of oil paintings for $9.99 (£7.64), which she intended to paint over. Thankfully, before Feeback attempted to paint over one of the pictures, a knowledgeable friend recognised it as the work of abstract artist Ilya Bolotowsky. Feeback put the painting up for auction at Sotheby's not long after, where it fetched more than $34,000 (£26,000).

Ian and Lynda Spires of Milton Keynes, England were clearing out their attic one day in 2009 when they noticed that the floorboards had the names of towns and cities written on them. The boards were actually valuable railway station signs, which went on to fetch $2,600 (£2k) at auction.

A Californian family cleared up in the money stakes after clearing out their late great-grandpa's house. A family member stumbled upon a tatty brown paper bag and decided to have a look inside – the bag contained seven super-rare Ty Cobb baseball cards dating from 1901, worth a staggering $1 million (£765k).

In 2008 Philadelphia native Norma Ifill visited an Alexander Calder exhibition and realised a necklace she had at home was one of the renowned American sculptor's missing works – it fetched $267,750 (£205k) at a Christie's auction in 2013. Ifill had bought this striking but 'cheap' necklace at a flea market in New York for just $15 (£11.50) three years earlier.

A Fillipino fisherman found the world’s biggest pearl, weighing in at 34kg, off the coast of Peurto Princesa, but was completely unaware of its value. The fisherman kept the clam pearl for a decade as a ‘good luck charm’, before passing it onto his aunt, a tourism officer, who found it to be valued in excess of $100 million (£76.5m). It certainly turned out to be a very lucky charm!

The Uncle Sam diamond was the largest diamond ever discovered in America back when W.O. Basham stumbled upon it when walking through what is now known as the Crater of Diamonds State Park in Arkansas. It weighed a massive 40.73 carats, but was later cut down to 12.42 carats and sold for $150,000 (nearly $1 million/£760,000 in today's money) in 1971.

A painting by the old master Jacob Jordeans, valued at $3.76 million (£2.87m), was discovered sitting in a storeroom in Swansea, Wales where it had sat for decades.

An amateur collector came across this beautiful metallic Chinese bowl in a charity shop in Somerset, England. The antiques enthusiast paid just $2.60 (£2) for the 18th-century bowl, which was later valued at $6,535 (£5,000). It went on to sell at auction for a very respectable $27,450 (£21,000).

An 18th-century bust of British politician Sir John Gordon was discovered propping open the door to a shed in the village of Balintore, Easter Ross in Scotland. Valued at more than $1.96 million (£1.5m), it has now been displayed in the Louvre.

During a shopping trip in 2014, Sean and Rikki McEvoy of Asheville, North Carolina paid $58 (£44) for a vintage sweatshirt from their local Goodwill store. Not long after, the couple were watching a documentary on legendary NFL coach Vince Lombardi and spotted him wearing the sweatshirt they'd bought – it sold at auction a year later for $44,150 (£33,775).

This dress, worn by Princess Diana to a state dinner in Bahrain in 1986, was found in a charity shop in Herefordshire in 1994 by a local woman. She bought it for just $261 (£200), paying in four $65 (£50) installments. The silk dress was designed by David and Elizabeth Emanuel was discovered and went to auction in December 2018, selling for $204,000 (£156k), surpassing its $78-$130,000 (£60-£100k) valuation.

In 2017 Hugo Sanchez stumbled upon one of only two Ford Mustangs custom-made for the classic film Bullitt, which boasts one of the most awesome car chases in history, in a junk yard in Mexico. The unexpected treasure has an estimated value of at least $1 million (£765k).

This still-life was purchased for just $30 (£23) in the 90s by a factory worker from Indiana, who used it to cover a hole in a wall in his home. Several years down the line, the hard-up machinist was playing the art-based card game Masterpiece and spotted his painting on one of the cards. The picture, which turned out to be a notable work by American landscape painter Martin Johnson Heade, went on to sell for $1.25 million (£956k).

A couple walking their dog on their land in California stumbled upon a rusty can sticking out of the ground. Inside were gold coins minted from 1847 to 1894 that, along with the other seven cans of coins they went on to find buried nearby, were worth over $10 million (£7.7m). Soon named the Saddle Ridge Hoard, the accidental discovery is made up of 1,400 gold coins.

Exceedingly valuable Ming vases masquerading as ordinary umbrella stands seem to be a thing. In 2010, a retired couple from Dorset, England brought their battered umbrella stand into Duke's auction house for valuation. It was revealed to be an 18th-century Ming vase worth $1.1 million (£841.5k).

Farmer David Mazurek had been using a 23-pound rock as a doorstop since 1988 when he bought a barn, and the rock came with it. He knew it was a meteorite, but had no idea of its value. It was only when he saw reports that meteorite pieces were selling for thousands he took it to the Central Michigan University for examination where Mona Sirbescu, a CMU geology professor, valued the rock at $100,000 (£76.5k). It was the most valuable specimen she had ever seen.

Sotheby's specialist Julian Gascoigne had the find of his career when he chanced upon this missing Constable oil painting hanging under the stairs in a client's home between two fairly ordinary canvases. As you can imagine, the client didn't waste any time putting the artwork up for auction, and in December, Sotheby's sold the lost painting for $2.1 million (£1.6m), eclipsing its estimate by a long shot.

The most important piece of Roman Baroque furniture ever sold in the UK, the bottom half of this ornate 17th-century cabinet was discovered by Sotheby's Mario Tavella in 2007 sitting outside the toilets in an Ask pizza restaurant in York.The cabinet, which was once owned by Pope Clement IV, was the star lot at Sotheby's Important Italian and Continental Furniture sale in December 2007, going under the hammer for just over $1.3 million (£1m).