You can now buy a $1,089 platinum bar from Costco—here's what to know before investing

Shoppers love Costco for the affordable gas, bulk-priced consumer staples and unwavering commitment to charging $1.50 for a hot dog and a soda.

Recently, though, they've found something new to love about the wholesaler: precious metals.

Costco began selling physical gold last year, and demand has been so hot that Wells Fargo analysts expect sales to now be in the neighborhood of $100 million to $200 million a month.

And earlier this month, the big box store began selling platinum on its website. A 1-oz. bar of the metal will run you $1,089, a small premium over platinum's spot price, which recently hovered around $1,000 an ounce. You can only buy the bars on the company's website, and you'll need a membership, which costs between $65 and $130 a year.

Costco's move into metals has made it easier than ever for everyday shoppers to own physical gold and platinum. But it's far from the first time precious metals have been in vogue, especially among people looking to invest in them.

Investors tend to flock to gold, in particular, in times of economic turbulence — especially amid fears that the fiat dollar may lose its value, due to inflation or other factors.

"Gold and other precious metals, over time, have a great record of protecting your purchasing power," says Trevor Yates, an investment analyst at ETF firm Global X.

Over the past 12 months, investors have piled into gold, driving its price up by 46% to about $2,700 an ounce.

Given that performance, "attention is turning to other assets, which have not fared as well and may offer value for investors," says Rob Haworth, senior investment strategist at U.S. Bank Wealth Management.

In other words, with platinum not having changed much in price during gold's recent rise, precious metals investors may be wondering if it's the next big thing.  

Platinum and gold share some of the same qualities, along with some key differences.

They're both precious metals that derive at least some of their value from their scarcity. There is only so much gold and platinum on this planet that humans will be able to mine. They're both coveted for their intrinsic beauty — both feature prominently in jewelry. And as investments, both tend to move differently than traditional assets, such as stocks and bonds.

Owning such assets — referred to as "uncorrelated" to movements in the rest of the market — can be a way to tamp down on risk in your portfolio by ensuring that not everything you own is moving in the same direction at once, says Yates.

"So maybe people are looking to diversify in gold and precious metals as a really uncorrelated asset class, which could add a lot of value to your portfolio," he says.

But potential investors in gold or platinum should know some key differences. For one, gold has a long history of value dating back to ancient civilizations and is still viewed as a stable asset. The bulk of its demand comes from institutional and retail investors.

Demand for platinum, meanwhile, is much more tied up in its usefulness as an industrial material. "Platinum is a key ingredient in traditional [internal combustion] and hybrid vehicles," says Yates. As a result, "demand for platinum is much more cyclical than the demand for gold."

What's more, while gold is largely mined on its own (sometimes alongside copper and zinc), platinum is often mined along with a group that includes palladium, rhodium and osmium.

Investors interested in platinum would be wise to "really pay attention" to the dynamics of all the platinum-group metals, Yates says, since fluctuations in any one of them can affect the bottom line of miners, which in turn, can affect platinum production.

How you invest in platinum, or any precious metal, depends on your reason for owning it.

If you're strictly looking for a portfolio diversifier, you may be in the market for a precious metals exchange-traded fund. These instruments hold a physical cache of precious metal and track the metal's spot price. Investors pay a small expense ratio and can own exposure to the precious metal of their choice in their brokerage account, right alongside their stocks and bonds.

Many investors would caution against putting a large portion of your portfolio in anything shiny, however.

"Our challenge is these assets do not provide an ongoing cash flow, such as income or dividends, to investors and require prices to rise over time for investors to garner value and a sale to capture value," says Haworth.

This sentiment echoes Berkshire Hathaway chairman Warren Buffett's argument against holding gold. In his 2011 shareholder letter, Buffett imagined owning all the world's gold in the form of a cube. In 100 years, he posited, you'd still own a cube.

With the same money, the thinking goes, you could buy income-producing assets, such as stocks, and be far, far richer in the long run.

Still, some investors see value in owning the real, physical thing, either because they're preparing for a day in which their dollar could be worthless, or because they distrust financial institutions to hold onto their metals for them.

If you're in that camp, and you want to own platinum, Costco offers a relatively attractive price from a trusted name, says Yates.

"One of the things that Costco does well is reducing the security component of buying and owning [precious metals]," he says. "It reduces the fear of fraud as well. Costco adds a lot of credibility."

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