For the ultra-rich, it involves investing millions in an original Warhol or Picasso. For others, it will be gold, silver or rare stamps. Economists are puzzling over how almost every type of asset - commodities, property, precious metals, shares, bonds and even art - are testing dizzying new price levels.

This parallel rise in all asset prices is unheard of in economic history. Traditionally, when commodities go up, stock markets go down. Stock markets don't like soaring commodity prices, which have in the past translated rapidly into higher inflation and higher interest rates.

But the link appears - for now - to have broken. This week global stock markets hit a new high, finally surpassing the "new economy" peak during the dotcom boom of 1999/2000. The Dow Jones average in New York rose again yesterday, and is just few points short of its all-time high of 11,722 on January 19 2000.

So, now a way has emerged for the more humble investor to make a killing on the international markets. Yesterday the price of copper hit an all-time record of $8,000 a tonne, driven by frantic buying and selling by commodity brokers and futures traders. But a little-noticed fact is that every 2p piece made before 1992 is 97% copper - meaning that each coin contains 6.9g of the metal. Collect together 145 of them, and you've got a kilo's worth of copper. Now, just find another 999kg, a total of 145,000 coins, and you've got a tonne.

On their face value, those coins are worth just £2,900. But taking them to a scrap merchant and selling them on the open market for their metal content will make you a cool £1,500 profit, especially if you throw in the 25kg of zinc that are also sitting in your goldmine of loose change. The same trick can be pulled off with pennies, so long as they were circulated before September 1992, when the Royal Mint introduced new 1p and 2p coins made from steel with only a thin copper plating. For a 50% return on an original investment, that's very good even for the likes of George Soros.

It's not just copper that is booming in price: gold this week broke through the $700-an-ounce level for the first time in 25 years, representing a 250% gain since 2001 alone.

In New York, auction houses are achieving breathtaking new prices for modern art, with the buyers believed to be Russian billionaires on an oil and commodity-fuelled spending spree.

At Christie's in New York on Tuesday night, a world record was set for one of Andy Warhol's Campbell's soup cans. The £6.3m paid for Small Torn Campbell's Soup Can (1962) was the highest ever for one of the images that heralded the birth of pop art. Works by Damien Hirst and David Hockney also achieved record prices, although nowhere near the £51.5m paid for Picasso's Dora Maar au Chat last week.

In the secretive art world, the identity of the Picasso buyer is unknown, but speculation is growing that it was a Russian steel magnate.

Just as puzzling is the behaviour of gold. Market folklore has it that gold goes up in price when investors are looking for a safe haven during choppy economic times, yet all the major global indicators - economic growth, inflation and unemployment - are relatively benign.

Meanwhile, the much-heralded UK property crash failed to materialise; instead the boom moved to the United States, where house prices have surged dramatically.

In the late 1990s market traders talked of the "goldilocks economy" of surging productivity and growth, to justify eye-watering valuations of some companies. Today the buzzword is the "super-cycle", an expansion in demand for commodities that could last for decades and which has at its core one factor: China. In the first quarter of 2006 China posted breakneck economic growth of 10.2%, sucking in commodities and guzzling petroleum on a phenomenal scale.

Low interest rates are the underpinning for the super-cycle, providing the cheap money for traders to speculate in assets. But yesterday Legal & General, one of the biggest investors on the London stock market, joined the growing chorus of voices predicting a soggy end to the boom.

It warned that global bottlenecks will stoke up inflation and force central banks, led by the US Federal Reserve, to raise interest rates and choke off the boom. Even in China wages (which are still only 3% of US labour rates) are beginning to rise strongly: in the Shenzhen region at the core of Chinese manufacturing, workers will receive a 23% rise in minimum wages in July.

Meanwhile, the message from art history is that when sudden wealth on a vast scale is poured into buying art, a slump won't be far away. In 1990 the buyers were Japanese billionaires and the target was Van Goghs. The Japanese industrialist Ryoei Saito stunned the art world by bidding $82.5m for Portrait of Dr Gachet. Just weeks later, when the Japanese central bank raised interest rates, the art market collapsed as the loans used to buy the art works went sour.

Portrait of Dr Gachet remains the art world's most enduring mystery. Saito died in 1996, and the whereabouts of the Van Gogh masterpiece is today unknown.