Two years into the worst slump in U.S. stock prices in a generation, few investors are holding out hope that Wall Street quickly will return to the heady levels it reached in the late 1990s.

If investors are holding out hope at all, it’s that history doesn’t repeat itself and that this bear market isn’t as painful and protracted as the decades-long downturn that followed the collapse of the Dow Jones industrial average in 1929 or the 13-year depression in share prices that followed the collapse of Japan’s Nikkei 225 Index in 1989.

Brokers talk in a passage between trading floors at the New York Stock Exchange. Investors have been adding less conventional assets, like real estate, gold and commodities, during the bear market on Wall Street.

The trouble is, it might.

So as they try to rebuild their retirement savings and bear-proof their battered portfolios, some investors are beginning to wonder if it isn’t time to add less conventional assets to the mix anything from greenbacks to gold coins, and from pork bellies to vintage Porsches.

“All clients want to talk about today are alternative asset classes,” said Maureen Gallogly, a portfolio manager at US Trust in Costa Mesa, Calif. “A few years ago, you couldn’t get anybody to talk about anything other than stocks.”

Don’t go overboard

In moderation, experts say it’s a healthy way to keep a portfolio in balance.

Bonds, of course, have always been the asset of choice for investors looking to diversify their stock-heavy portfolios. And during the past two years, as equities have swooned, bonds have benefited mightily.

But experts say many investors are beginning to look beyond bonds to other asset classes things like precious metals and other commodities, real estate, collectibles, even cash that they gave little, if any, consideration to during the stock-obsessed 1990s.

As long as investors don’t go overboard, swapping their portfolio of Cisco, WorldCom and Lucent shares for a portfolio of Baltic freight futures, Thomas Kincaid paintings and gold futures, this new interest in unconventional assets isn’t a bad thing, many experts say.

“If this market is good for anything because it’s certainly not good for our overall wealth it reminds us of an important long-term investment lesson: the importance of diversification, not only across individual stocks but also across asset classes,” said Richard Weiss, the chief investment officer at City National Investments in Beverly Hills, Calif.

Gold, real estate are hot

For some categories of alternative assets, one reason for their sudden popularity is their recent performance.

Gold prices, and gold mutual funds, have been on a tear this year. The top-performing mutual fund so far this year is the US Global World Gold fund, up almost 100 percent, according to Lipper.

Real estate, meanwhile, has been on a well-publicized tear.

Adding to the allure are the lessons of the not-so-distant past. During the last protracted market malaise in the United States, between 1966 and 1982, stocks and bonds both suffered as the dollar fell and inflation spiraled out of control.

At the same time, investments in natural resources and precious metals outperformed. Gold, for example, went from $35 an ounce in 1971 to $850 in 1980. Oil and natural-gas prices also shot up.

Although inflation remains tame at the consumer level, speculation that history may be about to repeat itself may help explain the performance of the Commodity Research Bureau index.

The price for this basket of 22 goods, ranging from sugar to burlap and from rubber to cocoa, has risen more than 10 percent this year the greatest first-half gain since 1988.

“Gold stocks and to a lesser extent oil stocks were safe havens the last time around,” said Alan T. Beimfohr, the president of Knightsbridge Asset Management in Newport Beach, Calif.

“If we knew it was going to be a repeat, I’d go out and buy our gold coins, Newmont Mining and collectibles, I suppose. Those would be the asset classes of choice.”

Keep stocks, bonds

Few of the experts suggest that these alternative assets should form more than 10 percent of your total portfolio. Stocks and bonds, they say, must remain your retirement workhorses.

The reason? Reliability and historical returns.

“With collectibles, you can never rely that there’s going to be a liquid market or even that they’re going to appreciate,” said Tom Lydon of Global Trends Investments in Newport Beach.

Another problem with collectibles, he said, is that the market often moves in tandem with the stock market, as flush investors bid up the price of everything from classic cars to baseball cards. As a result, it provides little protection.

Gold is more complicated. An investor who bought gold as a long-term investment in January 1980, when it reached $850 an ounce, has never recovered from the metal’s subsequent drop. Indeed, when adjusted for inflation, the CRB index has actually lost more than half its value during the past 20 years.

Even so, gold and other commodities have proved to be great short-term plays in the intervening years. In its most recent mini-rally, which has taken place since April, gold has risen almost 30 percent.