With stocks pricey and investors getting valuation jitters, precious metals may be just the thing to add a little luster to your portfolio.

For starters, there are reasons to believe that a bull market for gold and silver might be beginning.

Not long ago, gold and silver prices went through a rough patch. In September 2011 the price for gold hit nearly $1,900 an ounce before plummeting to $1,050 in December 2015, according to data from the London Bullion Market Association. Since then prices have risen slowly, with gold trading recently at about $1,250. Silver prices followed a similar pattern.

[Read: 7 Basic Materials Investments to Buy in 2017.]

The uptick in prices could continue because gold has a long history as the asset that investors flock to when everything else seems risky. And for some investors, the stock market is flashing a yellow warning light now.

“This is not a really good time to be buying equities,” says Don Coxe, chairman of Coxe Advisors LLC in Chicago, given their high price-earnings ratios.

A case in point is the cyclically adjusted price-earnings ratio, or CAPE, which is at 29.77, the highest it’s been since 2002. Indeed, in the past when the CAPE was this high, periods of much lower stock prices followed.

“You want to be buying the assets that are not equities, or the financial antimatter,” Coxe says. By financial antimatter, he means gold. Coxe worries that central banks, such as the Federal Reserve, have been printing so much money that inflation will rise, destabilizing the stock market.

Coxe likes gold because it has a limited supply. Although it doesn’t pay a dividend the way many stocks do, gold tends to hold its value over time.

Other experts also see gold’s potential to shine as signs of a slowing economy emerge.

“There is a huge divergence between soft and hard data,” says Ronald-Peter Stoeferle, managing partner and fund manager at boutique firm Incrementum AG in Lichtenstein.

Soft data, which reflect how investors or consumers feel about the economy, can be hard to quantify. Hard data, on the other hand, refer to quantifiable economic readings based on dollars or output.

For example, consider the discrepancy between consumer confidence (soft data) and auto sales (hard data). Despite dipping slightly in May, the Conference Board’s Consumer Confidence Index, which tends to bounce around a lot, is at levels not seen since just before the financial crisis. The index measures how people feel about the economy. Apparently, their optimism is running much higher than a few years ago.

Meanwhile, auto sales have declined steadily every month this year. Another hard data point: Sales of durable goods, such as dishwashers and refrigerators, have stayed stubbornly stagnant. Neither indicator points to a strong economy.

[See: 7 ETFs for a Solid Portfolio Defense.]

Worse, a sudden shock in the form of a trade war or a sizable increase in borrowing costs could plunge the economy into recession. That, at least, is a scenario Stoeferle considers plausible.

Which brings us back to gold.

“Recessions are a great environment for gold,” he says.

During the last two recessions, gold prices increased while the economy contracted, according to the MacroTrends website. When the U.S. entered a recession in March 2001, gold cost $263 a troy ounce. By November 2001, when the recession had ended, gold was $276. The price gain was even greater during the Great Recession. In December 2007, gold traded at $803, but by June 2009, when the recession ended, gold prices had risen to $935. So if Coxe and Stoeferle are right, owning some gold should pay off.

How much to buy. Because their prices aren’t correlated with stocks or bonds, precious metals can reduce a portfolio’s overall volatility or risk. That’s why many smart investors suggest holding 5 to 15 percent of a portfolio in gold and other precious metals, with some people believing that number should be higher.

The optimal amount of gold to hold has fluctuated between 27 and 30 percent since the late 1960s, says Jeff Christian, managing partner and founder of specialty commodities consulting firm CPM Group in New York. He estimates that total global gold holdings today amount to around 0.7 percent of assets, which means the world is massively underinvested in gold. If, collectively, investors upped their allocation to gold just a little, its price would jump.

Ways to invest. The SPDR Gold Shares exchange-traded fund (ticker: GLD), which holds bars of solid gold bullion, is the largest and most liquid of all the gold exchange-traded funds. Its annual expenses are 0.4 percent, or $40 dollars per $10,000 invested. For silver, the iShares Silver Trust ( SLV) has annual expenses of 0.5 percent, or $50 dollars per $10,000 invested. For other ETFs and mutual funds with exposure to gold and silver, see U.S. News Best Funds rankings for equity precious metals.

Investors can also buy physical gold, such as coins. The trick is to buy coins that are priced based on their precious metals content rather than those that have value based on their condition. The latter are known as numismatic coins, the former bullion coins.

American Eagles, in both gold and silver, are usually considered bullion coins. They are made by the U.S. Mint and distributed through coin dealers. Buyers should expect to pay a premium of a few percentage points above the gold price.

[See: 8 Gold ETFs to Buy Anytime.]

Although platinum and palladium are also precious metals, their markets are small and heavily influenced by the automobile industry, which uses the materials to make catalytic converters.

More from U.S. News

7 Stocks That Soar in a Recession

The Fastest Ways to Lose All Your Money in the Stock Market

8 Smart Ways to Invest in Metal Stocks

The Next Bull Market for Gold May Have Just Begun originally appeared on usnews.com