Forget gold. Classic cars are the new hot investment.

The KFLII is the cars, wine, furniture, etc. index. (Knight Frank)

For rich people seeking new places to put their money, maybe one more fun than mutual funds and less risky than tech startups, one asset class is beating out all the rest: Classic cars.

Real estate firm Knight Frank put out its Luxury Index on Friday, which tracks a basket of collectible items that serves as a good proxy for how good wealthy people are feeling about their situations in life. Known as the KFLII, it kept rising fairly steadily through the recession, and is now beating the global stock markets.

Not all collectibles are doing equally well, though. Antique furniture has been out of style for a while now, for example and fine art--usually a more volatile category--sank 6 percent this year. But prices for classic cars, as tracked by the Historic Auto Group, have more than doubled over the past five years, and grown by 28 percent in the last 12 months.

What's driving the bubble, if we might call it that? According to Knight Frank, cars are all the rage among Asian plutocrats. And unlike art, wine, and watches, there's more of a confined supply. The various indices only track certain models, and none of them are being made anymore (even so, it's hard to pin down the overall value of the market, since there's not a central collection point for that kind of information, like there is in the major stock markets). Hagerty breaks them down by type of car: The "blue chips" cover models made in the 1950s, 60s, and 70s. There are only so many of them, and even more people want to own one -- complete with fraying upholstery -- there's no way for the market to increase supply.

For comparables, Knight Frank plots the Luxury Index against something that's also known as a luxury commodity: Gold. The price of gold, though, has plummeted in recent months. That's probably a good sign for the global economy, since people don't feel the need to put their money in bars of something that's always seen as valuable even if currencies collapse.

Luxury items aren't really a substitute for gold, says Knight Frank analyst Andrew Shirley. That would be a better way to characterize equities, like stocks: When confidence in the economy is high, people put their money in riskier bets that have the potential to make large returns. Collectibles are where they put their money when they've made enough money in the stock market that they feel like diversifying -- or just having some bling to set them apart from the other millionaires.